AN ABSTRACT OF THE THESIS OF
Yousef Abdel Wahab El-Madhun
Soil Science
in
for the degree of
presented on
Doctor of Philosophy
June 6, 1984
Interaction of Selected Herbicides With Soil Constituents
Title:
Redacted for Privacy
Abstract approved:
Dr. VirgirK.Freed
Redacted for Privacy
// '
Dr. -J 1.0440 1 Young
Dr
The behavior of six herbicides in soil was studied under
controlled laboratory conditions.
Sorption of bromacil, chlorotoluron
and diuron was assessed by the batch equilibration procedure over a
broad range of aqueous phase sorbate concentration.
Linear equilibrium
sorption constants (Kp) were slightly higher at 4° C than at 25° C
and many-fold higher in the organic soil than in loamy sand soil.
By
referencing sorption to soil organic carbon content (0C), this vast
variation was reduced to less than 3-fold variation in K0p.
The
isosteric heats of sorption were exothermic and low suggesting an
Water solubilities (S) and octanol-water
entropy driven mechanism.
partition coefficients (K
ow
) of the compounds were determined and used
to derive equations for predicting Kpc.
These equations are useful in
estimating sorption of related pesticides and pollutants from knowing
their K
ow
and S.
Degradation of the three herbicides was followed by monitoring
14
CO
2
evolution from soil.
Temperature affected degradation more than
did soil type or initial herbicide concentration.
The degradation
was tripled with a 10° rise in temperature from 25 to 35° C.
The
presence of herbicides did not affect the activity of soil microorganisms indicating that degradation was a co-metabolism process.
Degradation fit first-order kinetics with no lag phase and with
minimum half-lives of about 200 days for diuron and chiorotoluron and
2600 days for bromacil.
The binding of these three herbicides plus diquat, glyphosate and
simazine with water-soluble soil organic materials (WSSOM) and soil
humic acids (HA) was studied by gel filtration chromatography.
The
chromatographic method adapted by Hummel and Dreyer (1962) was very
successful in proving the existence of binding.
Noted differences in
the elution behavior of herbicides were related to their physical and
chemical properties and their interaction complexes.
Infrared (IR) spectroscopy showed the involvement of ion exchange
reactions in diquat and H-bonding in diuron, chiorotoluron and bromacil
binding with WSSOM and HA.
Shifts in some characteristic IR absorption
bands suggested the formation of charge transfer complexes in WSSOM and
HA interactions with bromacil, chlorotoluron and diuron but not with
diquat.
The results appear significant in evaluating pesticides and
pollutants mobility and transport in soil and water environments.
Interaction of Selected Herbicides
with Soil Constituents
by
Yousef Abdel Wahab El-Madhun
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Completed June 6, 1984
Commencement June 1985
APPROVED
Redacted for Privacy
.
Professor of Soil Sciencein charge of major
Redacted for Privacy
Professor OLSoil-S\ciencelin charge
Redacted for Privacy
Head of epartmet of Soil Science
Redacted for Privacy
Dean of Graduate S
ool
/
Date thesis is presented
June 6, 1984
Typed by Becky Caldwell for
Yousef A. El-Madhun
ACKNOWLEDGEMENT
I wish to express my sincere appreciation to Dr. V. H. Freed and
Dr. J. L. Young, my co-major professors for their friendship, advice,
and guidance in conducting the study and in preparation of the thesis.
Special thanks go to Dr. S. C. Fang for his advice and use of his
laboratory space and facilities.
Sincere appreciation is extended to Dr. J. Baham, Dr. A. P.
Appleby, and Dr. I. J. Tinsley for their helpful comments and for
serving on my program committee.
Thanks are due to the staff of the Agricultural Chemistry Department for their friendship and support.
I am very grateful to my parents, my wife Jamila, and my daughter
Nassrene for their support and encouragement which made this period
of graduate study possible.
I am also grateful to my brothers in Salman Alfarici Islmaic
Center for their friendship and moral support.
Sincere thanks also go to El-Fateh University in Tripoli, Libya
for financing my graduate studies through the scholarship award.
Appreciation is due to Mrs. Becky Caldwell for typing this thesis.
TABLE OF CONTENTS
INTRODUCTION
1
CHAPTER I
5
SORPTION OF BROMACIL, CHLOROTOLURON AND DIURON BY
SOILS
Abstract
Introduction
Materials and Methods
Results and Discussion
Sorption Isotherms
Sorption Dependence on Soil Organic Carbon
Effect of Temperature
Sorption Dependence on Sorbate Properties
Summary and Conclusion
Acknowledgement
Literature Cited
6
8
13
17
17
20
22
23
27
29
42
CHAPTER II
BINDING OF SIX HERBICIDES WITH SOIL HUMIC ACID
Abstract
Introduction
Materials and Methods
Results and Discussion
GeT Filtration Chromatography
Infrared Spectroscopy
Acknowledgement
Literature Cited
48
49
50
CHAPTER III
73
ASSOCIATION OF SELECTED HERBICIDES WITH WATER
SOLUBLE ORGANIC MATERIALS FROM SOIL
Abstract
Introduction
Materials and Methods
Results and Discussion
Gel Filtration
Infrared Spectroscopy
Acknowledgement
Literature Cited
CHAPTER IV
DEGRADATION OF DIURON, CHLOROTOLURON AND
BROMACIL BY SOILS AT THREE TEMPERATURES
Abstract
Introduction
Materials and Methods
Results and Discussion
Literature Cited
52
55
55
58
62
70
74
75
79
82
82
85
90
101
106
107
108
111
113
128
SUMMARY AND CONCLUSIONS
131
BIBLIOGRAPHY
134
APPENDICES
148
LIST OF FIGURES
Figure
1.
Page
Structural formulae of the herbicides bromacil,
4
diuron, chlorotoluron, diquat, glyphosate and
simazine.
2.
Fitted isotherms for the sorption of chlorotoluron by
35
the peat soil at 4 () and 25° C (0).
3.
Representative isotherms for the sorption of
36
chlorotoluron by the peat soil at 4 () and
25° C (o).
4.
Representative isotherms for the sorption of diuron
37
by the peat soil at 4 (0) and 25° C (A).
5.
Representative isotherms for the sorption of bromacil
38
by the peat soil at 4 (0) and 25° C (0).
6.
Sorption isotherms for diuron by the loamy sand
39
soil at 4 (o) and 25° C ().
7.
Sorption isotherms for chlorotoluron by the loamy
40
sand soil at 4 (0) and 25° C (o).
8.
Freundlich isotherms for the sorption of diuron by
41
soil at 4° C (A, peat; 0, sand) and 25° C
(A, peat; , sand).
9a.
Elution profile for the measurement of diquat
binding by humic acid on Sephadex G-25 gel.
64
Figure
9b.
Page
Elution profile of diquat-HA binding (
65
superimposed on the elution profile of humic acid
(0; right-Y-axis) on Sephadex G-25 gel.
10.
Elution profile of diuron-HA binding (®)
66
superimposed on elution curve of diuron (A;
right-Y-axis) on Sephadex G-25 gel.
11.
Elution profile for the measurement of simazine
67
binding by humic acid on Sephadex G-25 gel.
12.
Infrared spectra of (A) humic acid (HA), (B)
68
HA-diquat, (C) HA-bromacil, (D) HA-diuron,
and (E) HA-chlorotoluron.
13.
Infrared spectra of herbicides (A) diquat, (B)
69
bromacil, (C) diuron, and (D) chlorotoluron.
14.
Elution behavior of water soluble soil organic
91
materials (WSSOM) determined on Sephadex G-25 (m)
and G-10 (, right-Y-axis) gels.
15a.
Elution profile of WSSOM-bromacil (v)
92
superimposed on elution curve of WSSOM (,
right-Y-axis) determined on Sephadex G-25 gel.
15b.
Elution profile of WSSOM-bromacil (v)
93
superimposed on elution curve of bromacil
(, right-Y-axis) determined on Sephadex G-25 gel.
16.
Elution profile of WSSOM-simazine (0)
superimposed on elution curve of WSSOM (®,
right-Y-axis) determined on Sephadex G-25 gel.
94
Figure
17.
Page
Elution profile of WSSOM-diuron (o)
95
superimposed on elution curve of WSSOM (©,
right-Y-axis) determined on Sephadex G-25 gel.
18.
Elution profile of WSSOM-chlorotoluron (A)
96
superimposed on elution curve of WSSOM (0,
right-Y-axis) determined on Sephadex G-25 gel.
19.
Elution profile of WSSOM-glyphosate (0)
97
superimposed on elution curve of WSSOM (11,
right-Y-axis) determined on Sephadex G-25 gel.
20a.
Elution profile of WSSOM-diquat (o)
98
superimposed on elution curve of WSSOM (g,
right-Y-axis) determined on Sephadex G-25 gel.
20b.
Elution profile of WSSOM-diquat (0)
99
superimposed on elution curve of diquat (.,
right-Y-axis) determined on Sephadex G-25 gel.
21.
Infrared spectra of (A) WSSOM, (B) WSSOM-diquat,
100
(C) WSSOM-bromacil, (D) WSSOM-diuron, and (E)
WSSOM-chlorotoluron.
22.
Flow- through soil incubation system used for
122
monitoring degradation of herbicides.
23.
Cumulative percentage of
14
24.
CO2 evolved from
123
C-diuron-treated soils incubated at 25° C.
Cumulative percentage of
14
14
14
CO2 evolved from
C-bromacil-treated soils incubated at 25° C.
124
Figure
25.
Page
Effect of temperature on degradation of
125
chlorotoluron (0.005 pmol g-1) by the peat
soil
26.
(s, 25° C; 0, 30° C; , 35° C).
Effect of temperature on degradation of bromacil
(0.1 pmol g
-1
126
) by the peat soil (, 25° C;
*, 30° C; 6, 35° C).
27.
Effect of temperature on degradation of diuron
127
(0.005 pmol g-1) by loamy sand soil (,
25° C; 0, 30° C; A, 35° C).
Appendix Figures
Freundlich isotherms for the sorption of bromacil
149
by soil at 4° C (m, peat; 0, sand) and 25° C
(o, peat; , sand).
2.
Freundlich isotherms for the sorption of
150
chlorotoluron by soil at 4° C (, peat; ,
sand) and 25° C (o, peat; o, sand).
3.
Elution profile of blue dextran 2000 on Sephadex
151
G-25 gel for measurement of void volume of the
column.
4.
Elution profile of chlorotoluron () and diquat
152
(, right-Y-axis) determined on Sephadex G-25 gel.
5.
Elution profile of glyphosate (A) and simazine
(s, right-Y-axis) determined on Sephadex G-25 gel.
153
Figure
6.
Page
Elution profile for the measurement of bromacil
154
binding by humic acid on Sephadex G-25 gel.
7.
Elution profile for the measurement of
155
chlorotoluron binding by humic acid on Sephadex
G-25 gel.
8.
Elution profile for the measurement of glyphosate
156
binding by humic acid on Sephadex G-25 gel.
9.
Cumulative percentage of
14
14
CO2 evolved from
C-chlorotoluron-treated soils incubated at
25° C.
157
LIST OF TABLES
Table
Page
1.
Selected properties of the two soils studied.
30
2.
Linear sorption constants (K ) and r values,
31
P
K
oc
values and Freundlich constants (K
f
)
and n and r values for the sorption of bromacil,
chlorotoluron, and diuron by the two soils.
3.
Sorption constants (K ) of bromacil,
32
P
chlorotoluron, and diuron calculated for each
initial herbicide concentration studied at 25° C.
4.
Octanol-water partition coefficient (Kow) and
33
water solubilities (S) of bromacil, chlorotoluron
diuron measured at 4, 25, and 40° C.
5.
Determined or estimated parameters for the
34
herbicides bromacil, chlorotoluron, and diuron
at 25° C.
6.
Elution parameter (Kay) for the herbicide and
63
herbicide-HA peaks and troughs determined on
Sephadex G-25 gel.
7.
Percentage
14
Co2 evolved per week from
14C-diuron, 14C-chlorotoluron, and 14C-bromacil
treated-soil as affected by temperature and
herbicide concentration.
118
Table
8.
Page
The rate per week of CO2 production from log
119
soil at 25, 30, and 35° C.
9.
Half-life (t1/2) of diuron, chlorotoluron,
120
and bromacil in peat and sandy soils as affected
by temperature.
10.
Activation energy (Ea) for degradation of
121
diuron, chlorotoluron, and bromacil by soils.
Appendix Tables
1.
First-order rate constant (K) for degradation of
diuron, chlorotoluron, and bromacil by soils at
25, 30, and 35° C.
148
Interaction of Selected Herbicides
with Soil Constituents
INTRODUCTION
Modern agriculture has become increasingly reliant on the use of
pesticides.
The use of these chemicals has been dictated by the
growing worldwide demand for food and fiber.
Pesticides are vitally
important to increasing and protecting the quantity and quality of food
commodities and in providing the society with better health and higher
standard of living.
However, the proliferation of chemical usage has
elicited concern over the possible effect of these chemicals on the
environment.
The cause of this concern was the finding that many
pesticides have become widespread environmental contaminants (Freed
et al., 1977).
Since pesticides will continue to be used in the
production of food and fiber, more should be learned about their
transport in the environment, the relationship of their physicalchemical properties to transport, their partitioning properties, and
their persistence in the biosphere (Freed and Hague, 1975).
A
thorough knowledge and understanding of the soil processes affecting
pesticides is imperative, if methods for controlling pesticide
persistence and minimizing their undesirable effects are to be devised
(Kaufman, 1974).
The herbicides selected for this study were chosen because they
represent major herbicide groups and are widely used on agricultural
crops.
The structural formulae of the herbicides are shown in Fig. 1.
The information presented below on the use and application rates of the
herbicides was extracted from the Herbicide Handbook, WSSA (1983) except
2
where indicated.
Bromacil (5-bromo-3-sec-butyl -6-methyluracil) is a
substituted uracil herbicide.
It is a general non-selective weed killer
in non-agricultural areas where bare ground is desired.
At lower
application rates (1.8 to 7.2 kg/ha) bromacil is used for selective
control of annual and perennial weeds in certain deep-rooted crops
such as pineapple, alfalfa and citrus.
Both diuron [3-(3,4-dichloro-
pheny1)-1,1-dimethylurea] and chlorotoluron [3-(3-chloro-4-methylpheny1)-1,1-dimethylurea] are substituted urea herbicides.
At low
rates (0.7 to 7.2 kg/ha) diuron is used as a selective herbicide for
the control of germinating broadleaf and grass weeds in crops such as
cotton, grapes, apples, pears, and citrus.
kg/ha) diuron is a general weed killer.
At higher rates (4.5 to 18
Combinations of bromacil and
diuron are used to control weeds in citrus and on non-cropland areas.
Chlorotoluron is effective both as a residual soil-acting herbicide
and as a contact foliar-applied against annual grass and many broadleaved weeds of cereal crops.
It is used at 1.5 to 3 kg/ha on winter
cereals before or after emergence of crop (Farm Chemicals Handbook,
1979; Worthing, 1983).
Diquat (1,1-ethylene-2, 2-bipyridylium ion) is
completely soluble in water and rapidly absorbed by green plants.
Diquat is used as a non-crop weed killer, a general aquatic herbicide
and as a preharvest top killer or desiccant of seed crops.
[N- (phosphonomethyl) glycine] is very soluble in water.
Glyphosate
It is foliar-
applied herbicide, relatively non-selective, and very effective on
deep-rooted perennial species and on annual and biennial species of
grasses and broad-leaved weeds.
Simazine [2-chloro-4,6-bis-(ethylamino)-
s-triazine] is a member of the s-triazine group of herbicides.
It is
widely used as a selective herbicide for control of broadleaf and grass
3
weeds in corn, established alfalfa, citrus, olives and deciduous fruits
and nuts.
Simazine is also used for selective control of algae and
submerged weeds in ponds.
At higher application rates (>4.4 kg/ha)
simazine is used as a non-selective herbicide for vegetation control in
non-agricultural areas.
The present extensive use of these herbicides has resulted in a
great deal of interest in their fate and behavior in soil.
A literature
review pertaining to the subject matter covered is presented in the
appropriate chapters of this treatise.
Although diuron and diquat have
been quite extensively studied, very little is known about the dynamics
of chlorotoluron and glyphosate in the soil environment.
The published
data on all of these herbicides were determined using different methodologies and dissimilar soils.
The objectives of this study were:
(i)
To assess sorption of bromacil, chlorotoluron and diuron by soils
for concentrations approaching their aqueous solubilities.
(ii)
To determine the value and reliability of the octanol-water
partition coefficient and water solubility of herbicides in
predicting sorption.
(iii) To establish the use of gel filtration chromatography for studying
the binding of herbicides with water-soluble soil organic materials
(WSSOM) and humic acids (HA) extracted from soil.
(iv)
To characterize the binding mechanisms the herbicides undergo with
WSSOM and HA using infrared spectroscopy.
(v)
To assess the kinetics of degradation and estimate the half-lives
of diuron, chlorotoluron and bromacil in soil.
4
0
II
N-C-N
CI
OH,
CH3
N
a
2Br-
DI URON
DIQUAT
0
N-C-NrCH 3
'CH3
CH3
II
a
Br
0
H
CHLOROTOLURON
CH3
BROMACIL
0
CI
0
HO-C -CH2 -N-CH2 -P-OH
Ni N
C2 H5 11--LN
N -91-1-C2H5
-C2H5
011
GLYPHOSATE
SIMAZINE
Figure 1.
Structural formulae of the herbicides bromacil, diuron,
chlorotoluron, diquat, glyphosate and simazine.
5
CHAPTER I
Sorption of Bromacil, Chlorotoluron, and Diuron by Soils
Yousef A. El-Madhun, V. H. Freed, J. L. Young
1
2
,
1
and S. C. Fang
Contribution of the Departments of Agricultural Chemistry and Soil
Science, Oregon State University, Corvallis, Oregon 97331.
2
Research Assistant, Professor of Chemistry, Research Chemist ARSUSDA and Professor of Soil Science, and Professor of Agricultural
Chemistry, respectively.
6
ABSTRACT
Sorption of bromacil (5- bromo -3- Sec - butyl -6- methyluracil), chloro-
toluron [3-(3-chloro-4-methylpheny1)1,1-dimethylurea] and diuron [3(3,4-dichloropheny1)1,1-dimethylurea] by soils of greatly different
organic carbon content from aqueous solution was studied.
The sorptive
capacity of the two soils was assessed by the batch equilibrium
procedure over a broad range of aqueous phase herbicide concentrations
at two temperatures, 4 and 25° C.
Sorption isotherms exhibited good fit
to both the Freundlich adsorption equation and the modified Freundlich
equation (n = 1).
Although the highly significant (r > 0.992)
correlation coefficients from least-squares-fit appeared to reinforce
a common misinterpretation of linear relationships, the sorption
isotherms for nearly all the soil-herbicide combinations showed some
dependence on concentration thereby demonstrating their nonlinear
nature.
The sorption coefficients (K p) of chlorotoluron were over 10 times
higher than those of bromacil but less than one-half the corresponding
K
values of diuron.
The K
values were slightly higher at 4° C than
at 25° C and 40-200-fold higher for the organic soil than for the
loamy sand soil, hence reflecting sorption dependence on soil organic
carbon.
By referencing sorption to soil organic carbon, this vast
variation in K
was reduced to less than three-fold variation in Km.
The isosteric heats (AH ) of sorption as a function of amount of chems
ical sorbed were exothermic and relatively low suggesting the involvement of entropy driven low energy requiring mechanism.
Experimental
values of the water solubility (S) and octanol-water partition
7
coefficients (K ) were determined at 4, 25, and 400 C, for the three
ow
compounds.
Sorption constants (Km) estimated from the octanol-water
partition coefficients were in better agreement with the measured Koc's,
than their counterparts predicted from aqueous solubilities of the
compounds.
A fit of the experimentally determined Kow and Koc data
for bromacil, chlorotoluron and diuron yielded a highly significant
positive relationship (r = 0.999).
A highly significant negative
correlation (r = -0.995) was observed between water solubilities and
K
oc
values for these compounds.
These relationships should be useful
in predicting sorption (Koc) of related (neutral) compounds from aqueous
solution by soils when experimentally precise values are not required.
The advantage of these correlations is that sorption of many organic
pesticides and pollutants can be estimated from easily determined
(available) sorbate parameters, namely, octanol-water partition
coefficient (K
ow
) or water solubility (S) of the chemical in question.
Additional Index Words:
coefficient.
Aqueous solubility, octanol-water partition
8
INTRODUCTION
The phenomenon of pesticide sorption in soil has been invoked by
many investigators to explain differences in the biological activity
of a pesticide in different soils.
When a herbicide is applied to soil
a major part of it comes in contact with soil colloids.
Although
sorption may occur from vapor, sorption from solution is the most
significant in a soil-herbicide system.
Sorption has the effect of
reducing the concentration of chemical in solution.
Consequently any
concentration-dependent process is affected (Osgerby, 1973).
In other
words, sorption determines the chemical's ultimate fate in soil.
Sorption is one of the major factors affecting the chemical persistence,
biological activity and mobility in soil (Bailey et al., 1968; Haque
and Freed, 1974; and Osgerby, 1973).
This explains why this phenomenon
has been given great attention as reflected in the large body of
literature published in the last two decades (Banwart et al., 1982;
Haque and Cashow, 1971; McCall et al., 1980; Hance, 1965b; Hamaker and
Thompson, 1972; and Dao et al., 1982).
These studies have been for the
most part descriptive in nature, and as Wauchope and Koskinen (1983)
pointed out, little new theory has been added in the last decade or so
despite the enormous number of experiments conducted.
This is due in
most part to the variability and complexity of soils and the existence of
several hundred different herbicide chemicals.
Equilibrium sorption is usually described by an equation known as
isotherm.
An isotherm is measured at constant temperature and
represents a relation between the amount of chemical sorbed and the
chemical left in the solution at equilibrium.
Sorption data are usually
expressed in the form of a constant such as the Freundlich adsorption
9
constant (K ) or the simple proportionality constant (K p) which
f
represent the extent of sorption.
This constant is a characteristic of
the particular soil-herbicide combination and is related to the free
energy changes of sorption (Hague and Freed, 1974).
Two of the major factors known to influence sorption are soil
properties and the molecular characteristics of the compound under
consideration (Bailey and White, 1964).
Although the effect of a number
of soil properties including clay type and content, pH, and cation
exchange capacity have been demonstrated (Bailey et al., 1968; Lie et
al., 1970; Mustafa and Gramar, 1972), sorption of neutral organic
pesticides has been found in many studies to depend primarily on the
organic carbon content of the soil (Kozak and Weber, 1983; Nkedi-Kizza
et al., 1983; Gerstl and Yaron, 1983; Grover, 1975; Hance, 1965; Peck
et al., 1980; Hilton and Yuen, 1963; Hassett et al., 1980; Means et al.,
1980).
The minor role displayed by the mineral fraction is presumably
due to its strong dipole interactions with the highly polar water,
which precludes an effective association of neutral organic compounds
with these mineral components of the soil (Chiou, 1981).
Lambert (1968)
discussed sorption equilibria in terms of interaction with soil organic
carbon.
This work along with that of Hamaker and Thompson (1972) has
produced a sorption constant (K
oc
) based on the soil organic carbon
content instead of the total soil mass.
The approach used in putting
sorption on a uniform organic carbon generated a unique constant (Km)
which appeared dependent on the properties of the compound being studied
but was unaffected by the makeup of soil organic materials and was
independent of soil origin or type (Lambert, 1968; Chiou et al., 1979;
Osgerby, 1973; Hamaker and Thompson, 1972).
The approach permitted
10
estimation of sorption with respect to a single soil property, i.e.,
organic carbon content.
Numerous investigators have shown that sorption (Koc) of neutral
organic pesticides can be related systematically to a number of sorbate
properties including water solubility, parachor, and organic-aqueous
(e.g.
octanol-water) partition coefficient.
The parachor was the first
sorbate parameter correlated with Koc (Lambert, 1967).
The rationale
behind the use of parachor stems from its being an approximate measure
of the molecular volume of the chemical.
Hance (1969) modified Lambert's
relationship in order to take into account the influence of solutesolvent hydrogen-bonding.
These combined efforts marked the first step
towards establishing a quantitative relationship between soil sorption
and chemical structure.
Lambert (1968) also suggested that the role of
soil organic materials was similar to that of an organic solvent and
that partitioning of a neutral organic compound between soil organics
and water should correlate well with its partitioning between water and
an organic solvent.
Briggs (1973) had successfully derived a relation-
ship relating sorption in four soils of 30 nonionic chemicals to the
octanol-water partition coefficient (Kow).
The term partition coeffic-
ient expresses the equilibrium concentration ratio of an organic
chemical partitioned between an organic liquid (e.g.
(Chiou et al., 1977).
octanol) and water
More recently, a number of workers (Brown and
Flagg, 1981; Felsot and Dahm, 1979; Karickoff et al., 1979) reported
highly correlated semi-empirical equations associating sorption of many
pesticides from water to the octanol-water partition coefficients.
The
significance of these relationships is that for certain organic compounds
(nonionic herbicides) the distribution coefficients which described
11
sorption equilibria are predictable from a simple liquid-liquid
partitioning experiment.
Water solubility of an organic solute can be
regarded as partition of the chemical between itself and water (Briggs,
1981).
Thus, one would anticipate a correlation between partition
coefficient and water solubility (S).
Chiou et al. (1977) have shown
such a correlation between log Kow and log S.
Later, Chiou et al.
(1979) reported a similar relationship between sorption (Koc) of many
organic chemicals and aqueous solubility of the sorbate.
Empirical
equations similar to the one developed by Chiou et al. (1979) were
reported (Briggs, 1981; Mingelgrin and Gerst, 1983; Means et al., 1980;
Karickoff et al, 1979; Felsot and Dahm, 1979) which permitted estimation
of a nonionic compound's Koc from its water solubility.
The equations
relating Koc and aqueous solubility are sometimes modified by correcting
for the crystal energy contributions of solid compounds (Briggs, 1981;
Karickoff, 1981).
Sorption of herbicides from water onto soil is in most instances an
exothermic process.
A decrease in temperature is usually associated
with an increase in sorption (Freed et al., 1977).
In general, for
neutral organic compounds, the isosteric heats of sorption (CHs),
which measure the strength of sorption are relatively low, and increased
sorption is not associated with comparable increases in Ms (Chiou et al.,
1979).
This one type of interaction has been considered to be an entropy
driven process and refered to as hydrophobic or nonpolar sorption
(Hamaker and Thompson, 1972; Hassett et al., 1980).
The objectives of this study were:
(i) to examine the effect of
soil organic carbon content on the sorption of bromacil, chlorotoluron,
and diuron at concentrations approaching aqueous solubilities of those
12
compounds; (ii) to study the dependence of sorption on temperature; and
(iii) to evaluate the value and reliability of the octanol-water
partition coefficient and water solubility parameters in predicting
sorption of these hydrophobic herbicides.
13
MATERIALS AND METHODS
Two soil types differing greatly in organic carbon content were
obtained from the upper (0-20 cm) layer of the soil profile from two
locations in the state of Oregon.
Adkins loamy sand (coarse loamy,
mixed, mesic, Xerollic Camborthid) was collected from the dry region of
eastern Oregon; whereas, Semiahmoo mucky peat (Euic, mesic, Typic
Medisaprist) was collected from the wet area of western Oregon.
The
soil samples were air-dried, and ground to pass through a 60 mesh
(0.246 mm) screen.
Selected properties of the two soils are given in
Table 1.
Analytical grade and 2-
14
C-Carbonyl labeled bromacil and
14
C-
labeled diurop were supplied by E. I. de Pont de Nemours and Co.
14
The
C-labeled at the carbonyl carbon and analytical grade unlabeled
chlorotoluron were provided by Ciba-Geigy Corp.
Bromacil, diuron, and
chlorotoluron were more than 99% analytically and radiochemically pure
and their specific activities were 3.97, 4.22, and 30.6 pCi/mg,
respectively.
The purity of the analytical grade samples was confirmed
with thin layer chromatography, whereas purity of the 14C-labeled
materials was determined with a Packard Model 7200, Radiochromatogram
Scanner.
Consequently, the
14
C-labeled and the analytical grade
unlabeled herbicides were used without any further purification.
Herbicide sorption on the two soils was determined using the batch
equilibrium procedure with a 1:5 soil to solution ratio.
14
Solutions of
C-labeled and analytical grade unlabeled compounds were used to
obtain desired herbicide concentrations.
Aqueous solutions, containing
herbicide concentrations of 2, 5, 20, 50, and 100 pM were prepared for
all three herbicides.
Additional concentrations were:
six for
14
bromacil at 500, 750, 1000, 2000, 2500, and 3000 (780 ppm) pM; four for
chlorotoluron at 150, 200, 250, and 300 (64 ppm) TIM; and three for
diuron at 75, 125, and 150 (35 ppm) pM.
Concentrations were chosed such
that the lowest level for each herbicide was below the usual recommended
field rate of application and the upper level was slightly under the
aqueous solubility limit (at 25° C) of the chemical.
Five milliliters
of herbicide solution were added to 1 g of soil in 50-mL Corex glass
centrifuge tubes.
The tubes were subsequently screw-capped with Teflon-
coated caps and shaken on a mechanical shaker for 24 h at 25° C or at 4°
C in the later case the shaker was placed in a cold room maintained at
4 ± 1° C.
At the end of the 24 h equilibration period, the samples were
centrifuged at 755 RCF for 30 min in a Sorvall superspeed RC2-B
refrigerated centrifuge set at the appropriate temperature.
Following
centrifugation the clear supernatant was removed and a 1 mL aliquat was
transfered into glass scintillation vials where 10 mL of Handifluor
Liquid Scintillation Counting Fluid (Mallinckvodt, Inc.) were added.
Equilibrium concentrations of the herbicides were determined by
measuring the radioactivity in solution (Packard Model 3330 Tri-Carb
Liquid Scintillation Spectrometer).
Since the counting efficiency of
the machine changes with color change and other factors, an internal
14
C-labeled standard was counted in each experiment to correct for any
variation in counting efficiency.
The amount of herbicide sorbed by
soil was determined by subtracting the concentration of the compound
after equilibration with the soil from the initial herbicide concentration.
Triplicate samples were measured for each initial concentration;
untreated soil samples were included as blanks.
15
The octanol-water partition coefficients (Kow) of the three
herbicides were determined following the method described by Chiou et
al. (1977) with some modifications.
Known quantities of
14
C-labeled
compounds were dissolved in 5 mL of high purity n-octanol in a 50 mL,
screw-capped Corex centrifuge tubes.
When solution was achieved, 15
mL of distilled water was added to make the total volume in each tube
20 mL.
The tubes were allowed to equilibrate by shaking on a mechanical
shaker for 2.5 h.
RCF for 30 min.
The phases were separated by centrifugation at 1085
The octanol phase was removed and sampled for liquid
scintillation counting.
After complete removal of the water-octanol
interface, the water phase was also sampled and analyzed with liquid
scintillation spectrometer.
The Kow of each herbicide was calculated
as follows:
C
K
ow
where C
o
=
7
o
refers to the concentration of herbicide in the octanol
phase and C
w
refers to the concentration in the aqueous (water) phase
at equilibrium.
The octanol-water partition coefficients were deter-
mined in quadruplicate at 4, 25, and 40° C.
The aqueous solubilities of bromacil, diuron and chlorotoluron
were determined in quadruplicate at 4, 25, and 40° C.
The method
described by Hurle and Freed (1972) which consisted of equilibrating a
quantity of the compound with water and measuring the concentration at
the end of the equilibration period was followed with few modifications.
An excess of compound (
14
C-labeled) was added to 20 mL of distilled
water in screw-capped centrifuge tubes.
The solutions were equil-
ibrated for 24 h at the appropriate temperature as follows:
in tem-
16
perature controlled water-bath shaker set at 40° C; on a shaker at room
temperature 25° C; and on a shaker at 4° C in a temperature controlled
cold room.
At the end of the equilibration period, the samples were
centrifuged at 1085RCF for 1 h and aliquots were removed and analyzed
by measuring the radioactivity in liquid scintillation spectrometer.
The solubility of herbicide in water was therefore established by
measuring the solute concentration in solution after equilibration.
It
shomld be noted that not only were equilibration and centrifugation
conducted at the designated temperature but also the removal of aliquot
samples from the tubes was performed at the same temperature.
17
RESULTS AND DISCUSSION
Sorption Isotherms
Equilibrium sorption is described graphically by a sorption
isotherm, which is a plot of herbicide concentration in the sorbed
phase versus its concentration in solution.
Representative isotherms
obtained using the method of least squares fitting of the sorption
data are shown in Fig. 1.
Sorption of the three herbicides was well
expressed with the simple proportionality relationship:
Cs
Kp Ce
(1)
as described by Hamaker and Thompson (1972), where Cs is the amount of
herbicide sorbed per unit mass of soil in pmol/kg, Ce is the solution
concentration after equilibration in pM, and Kp is the linear partition
(sorption) constant.
The values of Kp, as determined from the slopes of
the isotherms, along with the corresponding correlation coefficients (r)
for each soil-herbicide combination are given in Table 2.
All
2
regression lines generated had a coefficient of determination (r ) of
0.986 or higher, which indicated excellent fit of the data by this
simple relationship, eq. (1).
The fact that the correlation coefficients
are all highly significant (r > 0.944) must be carefully interpreted,
since the only safe conclusion one can make from this finding is that a
linear trend may exist between C
and C
s
e
(McClave and Dietrich, 1979).
These results do suggest such a linear trend.
However, graphical
representation of the data reveal that curvilinearity exists in all
soil-herbicide combinations studied (Figs. 2-6).
These curved sorption
patterns were not surprising since high herbicide concentrations were
included.
Because soil is such a heterogeneous sorbent, nonlinear
18
isotherms are usually obtained even when relatively very low herbicide
concentrations were used (Wauchope and Koskinen, 1983).
The upward bent
in the isotherms of diuron and chlorotoluron on Adkins loamy sand at 4°
C (Fig. 5 and 6) occurred when relatively very high solute (higher than
water solubility) concentrations were equilibrated with the soil.
For
the peat soil, with its very high sorptive capacity, the amount of
sorbate left in solution after equilibration was much less than the
aqueous solubility (x-axis, Figs.
observed even at 4° C.
1
to 4), hence no abnormal curving
The sandy soil on the other hand sorbed only a
small fraction of the total herbicide concentration applied, consequently,
concentrations in excess of the solute water solubility remained in the
equilibrium solution (x-axis, Figs. 5 and 6).
This unusual upward
deflection in the isotherms demonstrates the effect of temperature on
the aqueous solubilities of organic solutes, and strongly suggests the
presence of a crystalline phase in the equilibrium solution rather than
increased sorption.
Similar deviations (experimental errors) were noted
by Karickoff et al. (1979) for the sorption of the hydrophobic compound
pyrene on natural sediments.
The dependence of Kp, the sorption constant, on herbicide concentration can be assessed from equation (1).
C /C
s
e
The calculated Kp (K
=
) values, given in Table 3 for each herbicide concentration
studied, show a rather consistent decrease in Kp as the concentration
applied increased.
These data further demonstrate the nonlinear nature
of sorption and suggest that Kp may be considered constant for a narrow
concentration range only.
The data also show that the amount of
herbicide sorbed by soil continued to increase with each increase in
solute concentration but at a decreasing rate.
19
An examination of the data revealed that the results can be well
described in terms of the empirical Freundlich equation, which may be
written in the logarithmic form as follows:
log Cs = log Kf + n log Ce
where K
f
(2)
and n are constants representing the intercept and the slope
of the isotherm, respectively.
It is important to note that Freundlich
adsorption equation is equivalent to equation (1) when the exponent n
is equal to one.
Representative Freundlich isotherms obtained are
shown in Fig. 7.
Freundlich constants, Kf and n and the corresponding
r values (Table 2) were determined using the method of least squares
fit.
The high correlation (r > 0.993) reflects the fit of the data to
the Freundlich adsorption equation.
The Freundlich constants Kf and n
provide rough estimates of the sorbent capacity and the nature of
sorption, respectively (Adamson, 1976: page 389).
The n values were
nearly constant (0.85 - 0.97) and all were less than one which again
indicate the nonlinear nature of the isotherms (Hamaker and Thompson,
1972; Rao and Davidson, 1979).
Isotherms where n < 1 were attributed to
sorbents having higher affinity for the herbicide than the solvent,
water in this case (Freed and Hague, 1973).
The intercept, Kf, of
Freundlich isotherm provides some indication regarding the extent of
sorption and it is valuable for comparing sorption of different kinds of
sorbates on various sorbents.
that K
f
It is important to emphasize, however,
is a unit quantity estimated for a certain range of solute
concentration (Bowman, 1982).
Therefore, Kf should not be used before
knowing the units of determination and the range of concentration.
The
K values of a given herbicide are similar in value to the linear
f
sorption constants (K p) of the respective soil and temperature (Table 2).
20
Both show clearly the effect of herbicide nature, soil properties and
temperature on the process of sorption.
The linear sorption constants
are used here-in-below instead of Freundlich constants as the variation
in sorption is expressed in one constant rather than two constants (Kf,
n).
Sorption Dependence on Soil Organic Carbon
Inspection of the data in Table 2 shows that for a given chemical,
the average sorption constant of the organic soil is manyfold that in
the loamy sand soil.
This great difference between the two soils is,
of course, an expression of the high positive correlation between
sorption and the soil organic carbon content that has been reported by
many investigators (Felsot and Dahm, 1979; Khan et al., 1979; Gerstl
and Yaron, 1983; Means et al., 1980; BanWart et al., 1982; Zierath et
al., 1980; Karichoff et al., 1979; Hassett et al., 1980; Grover, 1975;
Liu et al., 1970; Hilton and Yuen, 1963; Hance, 1965).
Hamaker and
Thompson (1972) introduced a relationship for calculating sorption
constants based on soil organic carbon content instead of the total
soil mass.
This relationship is expressed by dividing the sorption
coefficients, Kp, by their respective organic carbon (OC) contents:
Koc
The K
(3).
oc
Kp/(% OC/100)
(3)
values contained in Table 2 were calculated from equation
It is obvious that the K
oc
values for a given herbicide are much
less variable among the two soils than are the corresponding constants
expressed on a total soil weight basis.
variation in K
In each case, a 40 to 200-fold
was reduced to less than three-fold variation in Koc
by referencing sorption to organic carbon.
These findings are in good
21
agreement with the data reported by Karickoff (1981), Rao and Davidson
(1979).
Thus putting sorption data on a uniform organic carbon basis has
the effect of yielding nearly constant Koc independent of soil origin
or soil type (Lambert, 1968).
Viewing sorption in this way incorpor-
ates two general assumptions.
The first assumption is that all soil
organic carbon is equally effective in sorbing herbicides, and the
second, is that mineral components in the soil are not significantly
active in the sorption process.
The first assumption is not always
correct (Mingelgrin and Gerstl, 1983) and as Hamaker and Thompson
(1972) noted that variability in Koc values from soil to soil may be
due to variations in the composition of soil organic materials.
In
spite of the highly positive correlations with organic carbon content,
contributions to the total sorption could come from soil mineral
fractions.
It has been shown by a number of workers (El-Madhun, 1979;
Felsot and Dahm, 1979; Hilton and Yuen, 1963) that there is a significant sorption of many neutral compounds after soil organic carbon has
been removed by wet oxidation or ignition.
For all practical purposes,
however, the two assumptions seem to hold fairly well for the sorption
of nonionic organic pesticides from water by soils used in this study.
Sorption coefficients (Koc) normalized on organic carbon basis are
better suited for comparison purposes than their counterparts (K ).
The K
oc
values obtained in this study for the three herbicides compare
well with the published data put on organic carbon basis Liu et al.,
1970; Grover, 1975; Hance, 1976; Rhodes et al., 1970; McCall et al.,
1980; Hance, 1965; Nkedi-Kizza et al., 1983; Majka and Lavy, 1977).
22
Effect of Temperature
The effect of temperature on the sorption of bromacil, chlorotoluron, and diuron on both soils studied is shown in Figs.
7 and in Table 2.
An increase in temperature from 4° C to 25° C
resulted in a decrease in K
small.
1 through
oc
values.
The decreases however are rather
Similar results were reported on the sorption of diuron,
bromacil and related compounds (Hague and Coshow, 1971; Liu et al.,
1970; Hurle and Freed, 1972; Peck et al., 1980).
From the sorption data
at 4 and 25° C the isosteric heat of adsorption, AHs, was calculated
using the expression described by Haque and Coshow (1971).
AHs = R (3 in Ce/3 1/T)c
where C
e
(4)
is the equilibrium concentration of the herbicide at a fixed
amount of herbicide sorbed (Cs), T is the absolute temperature, and R
is the gas constant.
The AH
values of chlorotoluron estimated at
s
different C
values were nearly constant ranging from -2.39 to -3.01 K
s
Cal mol
-1
for the two soils.
For bromacil the values of AH
s
became
less negative with increasing the amount sorbed in both soils which
agrees with the data reported by Hague and Coshow (1971).
In the case
of diuron, the isosteric heats of adsorption on sand were slightly
higher than their counterparts on the peat.
This was attributed to a
stronger bonding by the sand (Freed and Hague, 1973).
The isosteric
heats of sorption calculated as a function of chemical sorbed (Table
5) are consistent with the generalization that sorption of herbicides
onto soils is an exothermic (negative AHs) process.
The AHs values
are relatively small ranging from -1.58 K Cal mol-1 for bromacil to
-2.93 K Cal mol
-1
for diuron.
By inference, therefore, one can
23
speculate that the sorption mechanism for these herbicides is of the
type which involve electrostatic or entropy-generating forces but
not chemisorption.
Sorption Dependence on Sorbate Properties
Water solubilities (S) and octanol-water partition coefficients
(K
ow
) have recently been used to study sorption by soil of nonionic
organic compounds from water (Briggs, 1981; Chiou et al., 1979;
Karickoff, 1981).
K
ow
In this study, these two sorbate properties (S and
) were experimentally determined at 4, 25 and 40° C (Table 4).
measured aqueous solubilities at 25° C of 775, 81, and 40 mg L
-1
The
for
bromacil, chlorotoluron, and diuron, respectively, are in good
agreement with the data reported by Gunther et al. (1968) and the
manufacturers i.e. 815 and 42 mg L
respectively and 70 mg L
-1
-1
at 25° C for bromacil and diuron
for chlorotoluron at 20° C.
The aqueous
solubilities determined were used to calculate the heats of solution,
AH
w
for the herbicides using the following Clapeyron-type equation:
-AH_
1
In S =
(+)
(5)
where R is the gas constant, T is the absolute temperature in degrees
Kelvin, and S is the water solubility (mol L-1).
The resulting AHw
values (Table 5) compare well with the results of Hurle and Freed
(1972), and are consistent with the generalization that low water
solubilities of organic compounds are usually associated with positive
AHw (Chiou, 1981).
A comparison between the heats of solution, AHw,
and the heats of sorption AHs reveals that AHs is less negative than
-AH
w'
that is -AH
w
for all three herbicides.
> AH
s
These sort of data
24
together with low exothermic and nearly constant AHs values over a wide
range of solute concentration led Chiou et al. (1979) to suggest that
sorption of neutral organic chemicals by soil organics is mainly a
process of solute partitioning to the soil organic fraction.
These
investigators indicated that such results would not be compatible with
an adsorption model.
The results in Table 4 show that the octanol-water
partition coefficients (Kow) are less sensitive to temperature than the
water solubilities (S).
The change in Kow values was inconsistent and
fairly small, whereas the change in S was rather large and directly
proportional to the increase in temperature, that is as temperature
increases, S increases.
The octanol-water partition coefficients
determined in this study compare well with the published values of 257
and 479 for chlorotoluron and diuron, respectively (Briggs, 1981), and
70 to 80 for bromacil (Gerstl and Yaron, 1983).
As developed in previous sections, soil sorption coefficient
estimation for neutral organic chemicals can be reduced to estimation
of K
oc
because of the high correlation between sorption and soil organic
carbon content.
In recent years, several attempts have been made to
correlate sorption (Koc) of these compounds with their aqueous solubilities or their octanol-water partition coefficients.
Briggs (1981) and
Karickoff (1981) have reviewed the theoretical basis, in thermodynamic
terms, of the approaches used to predict Koc for the hydrophobic organic
compounds.
These and other workers (Chiou et al., 1979; Felsot and Dahm,
1979; Means et al., 1980; Brown and Flagg, 1981; Mingelgrin and Gerstl,
1983) have reported significant relationships between Koc and Kow and
between K
oc
and S, that allow the estimation of a compound's K
oc
value
from the knowledge of its octanol-water partition coefficient (Kow) or
25
its water solubility (S).
The range of predicted Koc values obtained by
using these relations to calculate Kocs of the three herbicides are
given in Table 5.
calculated K
Overall, the agreement between predicted and
values was good, generally agreeing within a factor of 3.
oc
This factor is comparable to the deviations in Koc values for a given
herbicide between the two soils (Table 2).
When the octanol-water
partition coefficients were used to predict Koc, nearly all values
obtained were in the range of the measured Kocs.
Water solubility on
the other hand was not such a good predictor of Koc as Kow.
This was
indicated by the range of nearly one order of magnitude in estimated
It has been pointed out that the correlation of Koc with S for
Koc*
solid compounds can be improved by including a term to account for the
energy needed to break up the crystal structure (Briggs, 1981; Chiou,
1981; Karickoff, 1981).
Since the octanol-water partition coefficient
involves the distribution of solute monomers between an aqueous phase
and a hydrophobic organic phase, it is not affected by the crystal
energy contributions (Karickoff et al., 1979).
Linear least squares fitting of the experimentally determined Kow
and K
oc
data for bromacil, chlorotoluron and diuron yielded the
following relationship:
r = 0.999
log Koc = 1.30 log Kow -0.77
(6)
whearas regression of the water solubilities (umol/L) and K
oc
gave
the equation:
log Koc = -0.80 log S + 4.41
r = -0.995
(7)
These correlations (equation 6 and 7) are similar to the equations
used above to estimate K
oc
from K
ow
and S values.
These highly
26
significant correlations show the validity of such parameters (K
and
0
S) to estimate sorption, Koc.
Hence, one might expect to estimate
fairly accurately the Koc values of other members of the substituted
urea herbicides or related compounds from equation 6 based on the
knowledge of Kow alone.
These relationships further demonstrate
the similarity between soil sorption of neutral organic compounds from
water and these chemicals partitioning between octanol and water
(Chiou et al., 1979).
Because the above correlations are generally
valid only for hydrophobic (neutral) compounds, this type of sorption
was refered to as hydrophobic sorption (Hassett et al., 1980) or as
solute partitioning to the soil organic fractions (Chiou et al., 1979).
Hydrophobic sorption was implied when a high degree of correlation
exists between chemicals
or S and sorption expressed on organic
K
ow
carbon basis (Hassett et al., 1981).
It is important to emphasize that although the three herbicides
studied agree very well with Koc-S relationships and with Koc-Kow
predictions, the limits of validity of these relationships are not
known.
Evidence in the literature (Hassett et al., 1981) suggests that
there is a limit beyond which these correlations may not hold and that
this limit is not strictly a function of the compound's water solubility
or its octanol-water distribution coefficient but is also a function of
sorbent properties.
This demonstrates the complexity of the sorption
process and suggests that the actual Koc may be estimated only within
and water solubility for members of diverse groups of
limits from K
ow
neutral organic compounds on all sorbents.
Experimentally determined
sorption isotherms are therefore needed for a precise knowledge of a
chemical's behavior in soil.
27
SUMMARY AND CONCLUSIONS
Sorption of three herbicides widely used in agriculture was studied
at two temperatures on two soils of greatly varying organic carbon
content.
The results were well described by a simple proportionality
relationship and Freundlich adsorption equation as indicated by the very
highly significant correlation coefficients of all isotherms.
Sorption
constants of a given herbicide were many times higher on the peat than
on the sand, reflecting the dependence of these chemicals sorption on
soil organic carbon content.
This wide variation was nearly eliminated
when sorption constants were computed on organic carbon basis instead of
total soil mass.
Although sorption was higher at 4° C than at 25° C,
the difference was only minor at this range.
Thus sorption is little
affected by temperature in the range common in temperate zone agricultural soils.
Isosteric heats of sorption, Ms calculated from sorption
data at different temperatures were exothermic and relatively low
suggesting the involvement of low energy (weak) mechanism.
The two sorbate parameters, aqueous solubility (S) and octanolwater partition coefficient (Kow) were experimentally determined for
each herbicide studied.
properties (S and K
ow
Sorption constants estimated from these sorbate
) using relationships from the literature developed
for similar compounds compared favorably with experimental values
normalized for soil organic carbon (Km).
It was shown that highly
significant linear correlations exist between the log values of both
K
ow
and S and the corresponding sorption constants (K
oc
).
Thus by
inference, it is concluded that such relationships may be useful in
predicting sorption of neutral organic compounds on a given sorbent in
28
instances when a reasonably accurate estimated value suffices in lieu
of a costly determined value.
29
ACKNOWLEDGEMENT
The authors with to thank Dr. J. Baham, Dr. A. P. Appleby, and
Dr.
I. J. Tinsley for their review of the manuscript.
Acknowledgement
is made to E. I. de Pont de Nemours and Co., and Ciba-Geigy Corp. for
supplying the herbicides.
The senior author wishes to express his
appreciation to the Libyan people and the University of Elfateh for
financing his graduate studies.
The organization and analysis of the
data base associated with this investigation were carried out in part
with the use of the PROPHET system, a unique national resource
sponsored by the Division of Research Resources, NIH, Bethesda,
Maryland 20014.
30
Table 1.
Selected properties of the two soils studied.
Soil
pH
0.M.
0.C.
Clay
Silt
Sand
16
65
19
3
13
84
%
Semiahmoo Mucky
5.4a
b
27.8e
37c
Peat
Adkins Loamy
48
7.3
0.7
Sand
a:
1:2 soil to water ratio
b:
ignition
c:
hydrogen peroxide oxidation
d:
Walkley-Black method
e:
% OM x 0.58
21.5
d
0.4
31
Table 2.
Linear sorption constants (K ) and r values, K
p
values
oc
and Freundlich constants (K ) and n and r values for the
f
sorption of bromacil, chlorotoluron and diuron by the
two soils.
Temp.
Soil
K a
c
b
r
oc
P
K
n
f
r
Bromacil
Peat
Sand
4
9.68
0.996
35
26.74
0.86
0.999
25
9.23
0.997
33
21.31
0.89
0.999
4
0.26
0.995
63
0.52
0.91
0.999
0.256
0.994
62
0.31
0.97
0.997
25
Chlorotoluron
Peat
Sand
4
103
0.997
370
158
0.85
0.999
25
75
0.997
269
116
0.87
0.999
0.66
0.999
163
1.47
0.86
0.995
0.5
0.99
122
1.08
0.87
0.999
4
25
Diuron
Peat
Sand
243
0.995
873
301
0.87
0.999
25
185
0.992
665
244
0.87
0.996
4
1.28
0.998
315
2.48
0.9
0.993
25
0.99
0.998
242
1.91
0.86
0.999
the linear sorption constant
a =
b = K
4
oc
c = K
f
= K /fraction organic carbon
p
Freundlich constant
32
Table 3.
Sorption constants (K p) of bromacil, chlorotoluron, and
diuron calculated for each initial herbicide concentration
studied at 25° C.
Bromacil
Init. Conc.
Chlorotoluron
Init. Conc.
K
P
K
P
PM
1AM
Diuron
Init. Conc.
K
P
PM
2
21.0
2
158.5
2
383.3
5
19.1
5
142.5
5
286.5
20
19.2
20
133.9
20
337.8
50
21.6
50
118.6
50
326.1
100
15.0
100
93.3
75
219.1
500
13.2
150
88.2
100
214.9
750
11.6
200
83.9
125
191.9
1000
11.3
250
77.0
150
191.4
2000
10.0
300
75.1
2500
9.2
3000
9.3
7
14.6
107.9
268.9
S
4.8
31.1
74.4
C.V.
33%
29%
28%
33
Table 4.
Octanol-water partition coefficient (Kow) and water
solubilities
S, of bromacil, chlorotoluron and diuron
measured at 4, 25, and 40° C.
Temperature
Bromacil
Chlorotoluron
Diuron
°C
4
69 ±
1
Kow
216 ± 1
490 ± 38
25
74 ±
2
219 ± 3
451 ± 25
40
80±
1
210 ± 6
423 ± 16
S (mg L-1)
4
25
40
626 ± 18
56 ± 6
20±
3
775 ±
8
81 ± 6
40 ±
1
1044 ± 35
99 ± 3
53±
1
34
Table 5.
Determined or estimated parameters for the herbicides
bromacil, chlorotoluron and diuron at 25° C.
Parameter
OHS
AH
K
a
w
oc
K
oc
K
a
AH
s
Chlorotoluron
Diuron
-1.58
-2.59
-2.93
2.32
2.73
4.86
determined
33-62
122-269
242-665
from K
48-97
124-286
180-475
50-665
212-2011
ow
from S
oc
Bromacil
and AH
w
are in K Cal mol
-1
.
323-3069
0* .
0
I 'f'f'111'1'
4
8
12
16
19
Ce (PM)
Figure 2.
Fitted isotherms for the sorption of chlorotoluron by the peat soil
at 4 (40) and 25 °C (o).
1500
rn
800
sew
'5
C.)
400
8
12
C
Figure 3.
16
19
(pM)
Representative isotherms for the sorption of chlorotoluron by the
peat soil at 4 (0) and 25°C (o).
750
1
1
1
1
1
V
1
I
1
I
10
600
OM.
/0
A
..........
70)
MI,
.bc
400
0
0
E
a
in
U
200
A
/ l'IIIII' I
1
2
.
I
3
,
.
4
Ce (pM)
Figure 4.
Representative isotherms for the sorption of diuron by the peat soil
at 4 (0) and 25°C (A).
10
8
6
4
//Mb
O
E. - . -
lib
.
r7
gh
2
C..)
0
0
r
.
I
200
.
I
'
400
600
800
100
Ce (1!)
Figure 5.
Representative isotherms for the sorption of bromacil by the
peat soil at 4 () and 25° C (0).
200
100
0
20
40
60
C
Figure 6.
e
80
100
('41)
Sorption isotherms for diuron by the loamy sand soil
at 4 (0) and 25° C ().
130
260
200
0
0
200
100
C
Figure 7.
e
(pM)
Sorption isotherms for chlorotoluron by the loamy sand
soil at 4 ) and 25° C (o).
280
-1.8
-0.8
0.2
1.2
Log Ce (0)
Figure 8.
Feundlich isotherms for the sorption of diuron by soil at
4° C (A, peat; o, sand ) and at 25° C (A, peat; , sand).
2.2
42
LITERATURE CITED
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1976.
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Soil Sci. Soc. Am. J. 46:740-
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Briggs, G. G.
1973.
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Briggs, G. G.
1981.
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J. Agric. Food Chem. 29:1050-1059.
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Brown, D. S. and E. W. Flagg.
1981.
Empirical prediction of
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Chiou, C. T.
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Partition coefficients and bioaccumulation of selected
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11.
Current Develop-
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Chiou, C. T., L. J. Peters, and V. H. Freed.
1979.
A physical
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Dao, T. H., D. B. Marx, T. L. Lavy, and J. Dragun.
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Effect,
and statistical evaluation, of soil sterilization on aniline and
diuron adsorption isotherms.
13.
El-Madhun, Y. A.
1979.
Soil Sci. Soc. Am. J. 46:963-969.
Fate of the herbicide diuron in soil.
M.S. Thesis, Washington State University, Pullman, WA.
14.
Felsot, A., and P. A. Dahm.
1979.
Sorption of organophosphorus
and carbamate insecticides by soil.
J. Agric. Food Chem. 27:557-
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15.
Freed, V. H., C. T. Chiou, and R. Hague.
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Chemodynamics:
transport and behavior of chemicals in the environment - a problem
in environmental health.
16.
17.
Freed, V. H. and R. Hague.
Environ. Health Persp. 20:55-70.
1973.
Adsorption, movement, and
distribution of pesticides in soils.
In W. Van Valkenburg (ed.):
Pesticides, Formulation, p. 441-459.
Marcell Dekker, New York.
Gerstl, Z., and B. Yaron.
1983.
Behavior of bromacil and napro-
44
pamide in soils:
Adsorption and degradation.
Soil Sci. Soc. Am.
J. 47:474-478.
18.
Grover, R.
on soils.
19.
1975.
Adsorption and desorption of urea herbicides
Can. J. Soil Sci. 55:127-135.
Gunther, F. A., W. E. Westlake, and P. S. Jaglan.
1968.
solubilities of 738 pesticide chemicals in water.
Residue Rev.
Reported
20:1-148.
20.
Hamaker, J. W., and J. M. Thompson.
1972.
Adsorption.
In C.A.I.
Goring and J.W. Hamaker (eds.) Organic Chemicals in the Soil
Environment, Vol. 1, p. 49-143.
21.
Hance, R. J.
1965a.
Marcel Dekker, Inc. New York.
The adsorption of urea and some of its
derivatives by a variety of soils.
22.
Hance, R. J.
1965b.
Weed Res. 5:98-107.
Observations on the relationship between
the adsorption of diuron and the nature of the adsorbant.
Weed
Res. 5:108-114.
23.
Hance, R. J.
1969.
An empirical relationship between chemical
structure and the sorption of some herbicides by soils.
Agric. Food Chem.
24.
Hance, R. J.
1976.
J.
17:667-668.
Adsorption of glyphosate by soils.
Pestic.
Sci. 7:363-366.
25.
Hague, R., and W. R. Coshow.
1971.
Adsorption of isocil and
bromacil from aqueous solution onto some mineral surfaces.
Environ. Sci. Technol. 5:139-141.
26.
Hague, R., and V. H. Freed.
environment:
1974.
Behavior of pesticides in the
"Environmental Chemodynamics."
Residue Rev. 52:
89-116.
27.
Hassett, J. J., W. L. Banwart, S. G. Wood, and J. C. Means.
1981.
45
Sorption of a-naphthol:
hydrophobic sorption.
28.
Soil Sci. Soc. Am. J. 45:38-42.
Hassett, J. J., J. C. Means, W. L. Banwart, S. G. Wood, S. Ali,
and A. Khan.
sediments.
29.
Implications concerning the limits of
1980.
Sorption of dibenzothiophene by soils and
J. Environ. Qual. 9:184-186.
Hilton, H. W., and Q. H. Yuen.
1963.
Adsorption of several pre-
emergence herbicides by Hawaiian sugar cane soils.
J. Agric.
Food Chem. 11:230-234.
30.
Hurk, K. B., and V. H. Freed.
1972.
Effects of electrolytes on
the solubility of some 1,3,5-triazines and substituted ureas and
their adsorption on soil.
31,
Karickoff, S. W.
1981.
Weed Res. 12:1-10.
Semi-empirical estimation of sorption of
hydrophobic pollutants on natural sediments and soils.
Chemosphere
10:833-846.
32.
Karickoff, S. W., D. S. Brown, and T. A. Scott.
1979.
of hydrophobic pollutants on natural sediments.
Water Res. 13:
Sorption
241-248.
33.
Khan, A., J. J. Hassett, W. L. Banwart, J. C. Means, and S. G.
Wood.
Sorption of acetophenone by sediments and soils.
1979.
Soil Sci. 128:297-302.
34.
Kozak, J., and J. B. Weber.
1983.
Adsorption of five phenylurea
herbicides by selected soils of Czechoslovakia.
Weed Sci. 31:368-
372.
35.
Lambert, S. M.
1967.
Functional relationship between sorption in
soil and chemical structure.
36.
Lambert, S. M.
equilibria.
1968.
J. Agric. Food Chem. 15:572-576.
Omega (c2), a useful index of soil sorption
J. Agric. Food Chem. 16:340-343.
46
37.
Liu, L. C., H. Cibes-Viadd, and F. K. S. Koo.
of ametryne and diuron by soils.
38.
Majka, J. T., and T. L. Lavy.
1970.
Adsorption
Weed Sci. 18:470-474.
1977.
Adsorption, mobility, and
degradation of cyanazine and diuron in soils.
Weed Sci. 25:401-
406.
39.
McCall, P. J., R. L. Swann, D. A. Laskowski, S. M. Unger, S. A.
Vrona, and H. J. Dishburger.
1980.
Estimation of chemical
mobility in soil from liquid chromatographic retention times.
Bull. Environ. Contam. Toxicol. 24:190-195.
40.
McClave, J. T., and F. H. Dietrich, II.
1979.
Statistics.
Dellen Pub. Co., San Francisco.
41.
Means, J. C., S. G. Wood, J. J. Hassett, and W. L. Banwart.
1980.
Sorption of polynuclear aromatic hydrocarbons by sediments and
soils.
42.
Environ. Sci. Technol. 14:1524-1528.
Mingelgrin, U., and Z. Gerstl.
1983.
Reevaluation of partition-
ing as a mechanism of nonionic chemicals adsorption in soils.
J.
Environ. Qual. 12:1-11.
43.
Mustafa, M. A., and Y. Gamar.
1972.
Adsorption and desorption
of diuron as a function of soil properties.
Soil Sci. Soc. Am.
Proc. 36:561-565.
44.
Nkedi-Kizza, P., P. S. C. Rao, and J. W. Johnson.
1983.
Adsorp-
tion of diuron and 2,4,5-T on soil particle-size separates.
J.
Environ. Qual. 12:195-197.
45.
Osgerby, J. M.
soil.
46.
1973.
Processes affecting herbicide action in
Pestic. Sci. 4:247-258.
Peck, D. E., D. L. Corwin, and W. J. Farmer.
desorption of diuron by freshwater sediments.
1980.
Adsorption-
J. Environ. Qual.
47
9:101-106.
47.
Rao, P. S. C., and J. M. Davidson.
1979.
Adsorption and movement
of selected pesticides at high concentrations in soils.
Water
Res. 13:375-380.
48.
Rhodes, R. C., I. J. Belasco, and H. L. Pease.
1970.
Determin-
ation of mobility and adsorption of agrichemicals in soils.
J.
Agric. Food Chem. 18:524-528.
49.
Wauchope, R. D., and W. C. Koskinen.
equilibria of herbicides in soils:
1983.
Adsorption-desorption
A thermodynamic perspective.
Weed Sci. 31:504-512.
50.
Zierath, D. L., J. J. Hassett, W. L. Banwart, S. G. Wood, and
J. C. Means.
1980.
Sorption of benzidine by sediments and soils.
Soil. Sci. 129:277-281.
48
CHAPTER II
Binding of Six Herbicides With Soil Humic Acidl
Y. A. El-Madhun, V. H. Freed, and J. L. Young
1
2
Contribution of the Oregon Agricultural Experiment Station.
Technical Paper No. 7085.
Departments of Agricultural Chemistry and
Soil Science, Oregon State University, Corvallis, Oregon 97331.
2
Research Assistant, Professor of Chemistry, and Research Chemist
ARS-USDA and Professor of Soil Science, respectively.
49
ABSTRACT
The binding of bromacil (5- bromo -3- sec - butyl -6- methyluracil),
chlorotoluron [3-(3-chloro-4-methylpheny1)1,1-dimethylurea], diuron
[3-(3,4-dichloropheny1)1,1-dimethylurea], diquat (1,1-ethylene-2,2bipyridylium ion), glyphosate [N-(phosphonomethyl) glycine], and
simazine (2-chloro-4,6-bis-ethylamino-s-triazine) with humic acid (HA)
from soil was studied by means of gel filtration chromatography and
The gel filtration procedure of Hummel and
infrared spectroscopy.
Dryer appears suitable for evaluating HA interactions with ionic and
nonionic herbicides.
The elution parameters (Kav) of herbicide-HA
peaks were indifferent of HA Kav of 0.07 and the elution parameters of
herbicide-HA troughs were approximately equal to the Kav's of the
herbicides.
Infrared spectroscopy was utilized to demonstrate the
involvement of ion exchange mechanism in the binding of diquat by HA.
Changes in the absorption bands of HA in the region 1720-1600 cm
-1
after treatment with bromacil, diuron, and chlorotoluron showed that
hydrogen bonding and possibly charge-transfer mechanisms were involved
in HA binding with these herbicides.
The results of this study could
have important implications in the assessment of pesticide mobility
and bioavailability in soil and water environments.
Additional Index Words:
gel filtration, infrared spectroscopy,
bromacil, diuron, chlorotoluron, diquat, glyphosate, simazine.
50
INTRODUCTION
The importance of soil organic materials with respect to
pesticide sorption has been discussed in several reviews (Hayes, 1970;
Khan, 1978; Stevenson, 1976; Burchill et al., 1980).
Studies with
simplified systems involving well-defined soil organic matter
components such as humic acid or fulvic acid have evolved over the last
decade (Khan, 1977; Burns et al., 1973b; Nearpass, 1976; Mueller-Wegener,
1977; Gaillardon et al., 1980; Senesi and Testini, 1980).
Techniques
used to study HA-herbicide interactions have included the slurry method
(Gaillardon et al., 1980; Khan, 1974), gel filtration (Burns et al.,
1973b; Grice and Hayes, 1970; Khan, 1973), and equilibrium dialysis
(Gilmour and Coleman, 1971).
The slurry method, which has been widely used by investigators,
can have a number of shortcomings in HA-herbicide binding studies.
It has been pointed out (Hayes, 1970) that this technique cannot be
satisfactorily applied to the study of binding by components which are
not sedimented by centrifugation.
The potential uses of gel filtration
techniques as an alternative for the study of binding of triazines by
soluble humic materials was outlined by Hayes (1970) and by Grice and
Hayes (1970) who suggested that the gel filtration method described by
Hummel and Dreyer (1962) would give a more quantitative evaluation of
the interactions between HA and herbicides in general.
This method
was successfully employed by Khan (1973) in studying the interaction
of bipyridylium herbicides with humic acid and fulvic acid using
Sephadex G-10 and G-25 gels.
51
Infrared spectroscopy has been used to obtain information on the
mechanism(s) of interaction between humic materials and herbicides.
This technique is of considerable value in HA-herbicide interaction
research since it yields a variety of bands diagnostic of specific
molecular structures and functional groups.
Infrared data provided
clear evidence for the involvement of ion exchange mechanism in the
binding of diquat and paraquat herbicides by humic acids (Khan, 1974;
Burns et al., 1973).
On the basis of infrared studies, Khan (1973)
suggested that the bipyridylium herbicides, diquat and paraquat, form
charge-transfer complexes with humic substances, a result that could
not be confirmed by Burns et al. (1973).
Infrared data provided
support for the belief that binding of s-triazines by HA involves such
mechanisms as ionic binding, H-bonding, Van der Waals forces and
charge-transfer complexes (Sullivan and Felbeck, 1968; Senesi and
Testini, 1980 and 1982).
Results on the binding of several substituted
urea herbicides by HA showed that besides the involvement of H-bonding,
a prominent role is played by electron donor-acceptor processes which
leads to the possible formation of charge-transfer complexes (Senesi
and Testini, 1980 and 1983).
In the study with infrared spectroscopy,
Khan and Mazurkewich (1974) failed to show the coordination of linuron
to cations on humic acid.
The objectives of this work were:
(i) to examine the suitability
of the Hummel and Dreyer gel filtration method for studying the
binding of ionic and nonionic herbicides by humic acid extracted from
soil, and (ii) to use infrared spectroscopy to characterize the binding
mechanisms these herbicides form with humic acid.
52
MATERIALS AND METHODS
Humic acid was obtained from Dr. R. L. Malcolm, USGS, Denver,
Colorado.
It was extracted from Fairbanks soil, Fairbanks, Alaska.
Malcolm (1976) described the methods of extraction and purification
which involved centrifugation, pressure filtration, dialysis, resin
exchange, and freeze drying steps.
The HA was H-saturated, and
contained on ash-free basis 57.13 % C, 34.45 % 0, 3.74 % H, 3.94 % N,
0.39 % S, and 0.13 % P.
The ash content was 0.38 %.
Six 14C-labelled herbicides were used:
diuron, simazine, diquat, and glyphosate.
bromacil, chlorotoluron,
Diquat and glyphosate are
both dissociable and can exist in ionic forms; whereas bromacil,
chlorotoluron, diuron, and simazine are nonionic and have limited
solubility in water at 25° C ranging from 775 mg/L for the fairly
soluble bromacil to 3.5 mg/L for the very low water soluble simazine.
All chemicals used were >99 % analytically and radiochemically pure.
The Hummel and Dreyer (1962) gel filtration procedure was used to
study the interactions between the herbicides and HA.
This method was
devised to detect interactions between macromolecules and substances of
low molecular weight.
It avoids the difficulties encountered in zonal
gel filtration due to interaction of ionic and aromatic compounds of
small molecular size with the Sephadex matrix.
It also eliminates the
problem of dissociation of the macromolecule-ligand complex in the
chromatographic column.
The Hummel and Dreyer method involves the
adding of a small volume of HA-herbicide mixture to a column of Sephadex
gel previously in equilibrium with a solution of the herbicide at the
same concentration as in the mixture.
When the column is eluted with
53
the equilibration solution, the elution profile shows a leading peak
which emerges from the column at the elution volume of HA, followed by
a trough which emerges at the elution volume of the herbicide.
The
peak and the trough are usually separated by a short region in which
the herbicide concentration returns to its base-line level.
The
appearance of a peak followed by a trough in the elution profile is
used as the criterion of binding.
The area of the trough provides a
direct measure of the number of moles of herbicide bound by HA.
Theoretically, the area of the peak should be equal to the area of the
trough (Wood and Cooper, 1970).
The gel filtration study was carried out by preparing,
in phos-
phate-buffer solution of pH 6.85 and ionic strength 0.05 M KC1,
herbicide solutions of the following pmolar concentrations:
bromacil,
0.966; chlorotoluron, 0.311; diuron, 1.01; diquat, 0.125; glyphosate,
1.01; and simazine, 0.109.
A known amount of HA (3 mg) was dissolved
in 3 mL phosphate-buffer-herbicide solution and applied to a 26 by 370
mm chromatographic column packed with fine mesh, Sephadex G-25 gel
(exclusion limit, 5000), which has previously been equilibrated with
the herbicide-buffer solution used to dissolve HA.
The column was then
eluted at 25 C with the same herbicide solution in the descending
direction using a peristaltic pump driven by a stepping motor at a flow
rate of 60 mL h
-1
.
Uniform fractions (approx. 5 ml), were collected in a fraction
collector.
The effluent solution was continuously monitored by ISCO
Model UA-2 Dual UV Analyzer set at 280 nm wavelength.
2000, at a concentration of 2 mg mL
-1
Blue Dextran
was used to determine the
uniformity of packing and the void volume of the column.
54
Herbicide concentration in each fraction was determined by taking
0.5 mL aliquots, adding 3.5 mL of Handifluor liquid counting solution
and measuring the radioactivity with a Packard Model 3330 Tri-Garb
Liquid Scintillation Spectrometer.
An internal standard was used
regularly to evaluate the instrument counting efficiency throughout the
course of study.
The elution profile of HA was determined by passing 3
mg HA through the column.
The concentration of HA in each fraction was
assessed by measuring the absorbance (optical density) at 400 and 280
nm on a Beckman Model 25 UV-Visible spectrophotometer.
Infrared (IR) spectra were recorded on a Perkin-Elmer Model 457
Grating Infrared Spectrophotometer in the 4000 to 400 cm-1 wavenumber
range using 1 mm thick (cell path) KBr pellets.
The IR spectra were
obtained for diquat, bromacil, chlorotoluron, and diuron at 1 mM
concentration.
Equal quantities of HA and herbicide:
7.2 mg diquat,
5.2 mg bromacil, 4.7 mg diuron, and 4.2 mg chlorotoluron were placed in
Corex centrifuge tubes and dissolved in 20 mL distilled water.
The
tubes were shaken on a mechanical shaker for 15 hr and the excess
moisture in the mixture was removed by freeze-drying.
The KBr pellets
were prepared by mixing 250 mg KBr with 1 mg of the freeze-dried
HA-herbicide mixture.
Recording conditions were:
slit, normal; and scan time, medium.
reference, air;
A reference spectrum of poly-
styrene film (0.05 mm cell path) was used to calibrate the spectra.
55
RESULTS AND DISCUSSION
Gel filtration chromatography
Representative elution diagrams for the measurement of binding of
herbicides by humic acid are given in Figs. 9, 10, and 11.
The elution
profile for diquat-HA system (Fig. 9a) showed the appearance of a peak
at the elution volume of HA and a trough at the elution volume of
diquat.
The existence of a trough at the elution volume of diquat
provided evidence for the binding between the herbicide and HA, as did
the presence of excess diquat in the HA peak (Hummel and Dreyer, 1962).
The attainment of equilibrium in the operation of the gel filtration
method was indicated by the return of the base-line concentration of
small molecule (herbicide) to its original level after the appearance
of the trough in the elution diagram.
This criterion was used to test
the adequacy of the method performance (Hummel and Dreyer, 1962).
The
results obtained on the binding of herbicides by HA provided indication
that equilibrium conditions were met satisfactorily in all systems.
Fig. 9b is shown the elution pattern of diquat-HA system superimposed
on the elution profile of humic acid.
This graphical presentation
shows the coincidence of the herbicide-HA complex peak on the humic
acid peak.
The results on diquat-HA binding determined in this study
were in essential agreement with the data reported by Khan (1973).
The elution profile for the measurement of diuron binding by HA
is shown in Fig. 10.
The trough in the diuron-HA system appeared at
the elution volume of diuron and the peak appeared at the HA elution
volume.
This diagram represents a typical elution pattern for the
In
56
binding of a small molecule with a macromolecule as described by Hummel
and Dreyer (1962).
The data for simazine, a triazine herbicide,
interactions with HA (Fig. 11) indicate that the gel filtration method
may be the technique of choice for measuring the complexation of these
herbicides with humic materials.
Similar results were obtained on the
binding of bromacil, chlorotoluron, and glyphosate by HA.
These data
demonstrate that the Hummel and Dreyer gel filtration procedure was
well suited for the study of binding ionic and nonionic herbicides by
humic substances.
Since the primary purpose of the present study was to examine the
utility of the gel filtration method to detect interactions between
herbicides and HA, no attempt was made to determine quantitatively the
amount of herbicide bound by HA.
The procedure, nevertheless, provides
a means of estimating extent of binding and gives data from which the
number of moles bound by HA can be determined by graphical (Hummel and
Dreyer, 1962) or mathematical procedures (Fairciough and Fruton, 1966).
Elution parameters (Kav) for the herbicides individually, and
when in association with HA, are presented in Table 6.
The Kav values
reflect the characteristics of the chemical and only chemicals that
have closely related properties can have similar Kav values.
This was
evident for the substituted urea herbicides, diuron and chlorotoluron,
and for the ionic herbicides diquat and glyphosate.
Elution para-
meters of herbicide-HA peaks were found to compare well with one
another and with HA Kav of 0.07, indicating that herbicide-HA
complexes have all appeared at the elution volume of HA.
Comparison
of the Kav values of herbicide-HA troughs with those of the herbicides
57
shows that these values were very similar.
These results are expected
whenever the method is adequately suited for the system and carried
out satisfactorily.
The data in Table 6 reveal the occurrence of
interactions between the Sephadex matrix and nonionic herbicides
(Kav > 1).
Evidently, the interactions had no effect on the
herbicides binding, except the delay of the trough emergence from the
column.
When the gel filtration procedure is carried out as originally
described by Hummel and Dreyer, the area of the trough should be equal
to the area of the leading ligand peak (ratio = 1).
The ratios of
trough areas to peak areas determined in this study were; 0.88, 1.28,
1.17, 0.97, 0.94, and 0.81 for bromacil, chlorotoluron, diuron, diquat,
glyphosate, and simazine, respectively, all within experimental error
from the theoretical value of 1.
Preliminary experiments with diuron, bromacil, and chlorotoluron,
utilizing Sephadex G-10 and G-25 gels showed that studying the binding
of these herbicides by humic materials was not possible by the zonal
filtration technique.
This was due to the dissociation of herbicide-
HA complexes as they passed down the chromatographic column.
These
observations indicated that the herbicides were weakly bound by a
reversible process to humic acid and suggested that the interactions
were physical in nature and may involve such mechanisms as H-bonding,
Van der Waals forces, and charge-transfer complexes (Wood and Cooper,
1970).
The Hummel and Dreyer (1962) gel filtration method can be used to
study the simultaneous binding of two herbicides.
This is possible
when different analytical procedures are available for each herbicide.
58
Different assay methods are not, however, necessary if the two
herbicides have elution volumes sufficiently different for their
troughs to be completely separated (Fairclough and Fruton, 1966).
Infrared Spectroscopy
Infrared spectra of untreated humic acid and herbicide-HA
complexes are given in Fig. 12.
Interpretation of the IR spectra and
band assignments were based on Bellamy (1975), Silverstein et al.
(1981) and Stevenson (1982).
All herbicide-HA spectra show a number
of changes in functional group absorption bands when compared with the
untreated humic acid spectrum, thus pointing distinctly to the binding
of the herbicides by HA.
The IR data of untreated HA (Fig. 12a) show characteristic
absorption bands of a typical HA preparation.
3300 cm
-1
The broad band around
was most likely due to N-H stretching and hydrogen bonded OH
groups of COOH and others.
The C=0 stretching vibrations of carboxylic
acids and ketones appeared at 1717 cm-1 in the IR spectrum.
absorption bands in the 1660-1600 cm
-1
The
region were attributed to
aromatic C =C vibrations and H-bonded C=0 groups of quinones and
ketones.
The band near 1400 cm
-1
was probably due to 0-H deformation
and C-0 stretching of phenols and asymmetric stretching of C00- ions.
The absorption band near 1200 cm
0-H bending of COOH.
-1
was assigned to C-0 stretching and
Although detailed examination of IR spectra of a
number of different humic acids has been made, it is still rather
difficult to unequivocally assign all absorption bands to specific
functional groups.
This could probably be due to overlapping of many
of the absorption bands (Burns et al., 1973) and interference caused
59
by adsorbed moisture which produces bands in the 3300-3000 and
1720-1500 cm-1 regions (Stevenson 1982).
The main changes in all herbicide-HA spectra occured in 1720-1660
cm
cm
-1
-1
region.
These effects, the reduced absorption frequency at 1717
and the enhanced intensity of the band in the 1660 to 1600 cm -1
region, can be explained in a number of ways with consideration of the
chemical nature of the interacting herbicide.
For the positively charged herbicide diquat, the changes are.
consistent with the conversion of HA carboxylic acids (COOH) to
carboxylate ions (C00) upon the addition of diquat to HA.
The
absorption band near 1610 cm-1 in diquat-HA spectrum (Fig. 12b)
indicate that carboxylate anions were produced when diquat was added
to humic acid and provide clear evidence for the involvement of
ion-exchange mechanism in this system.
These results are consistent
with those published by Khan (1974) and Burns et al. (1973a).
Khan
(1973) suggested the involvement of charge-transfer mechanism in
diquat and paraquat interactions with HA, based on the shifting of the
out-of-plane vibration bands of C-H groups to lower wavenumbers in the
900-600 cm
-1
regions.
Those shifts were not evident in this work nor
in the study of Burns et al. (1973a).
For the remaining herbicides, it seems unlikely that the changes
observed in the 1717 to 1600 cm
of COOH to C00
-1
can be explained by the conversion
as in diquat, because these herbicides are nonionic
and very weakly basic.
These absorption patterns can however be
attributed to any resonance effect that increases the C=0 bond length
and reduces the frequency of absorption.
Intermolecular H-bonding,
polar environment, delocalization of the 7 electrons of the C=0 group
60
could make the resonance effect to dominate, thus reducing the double
bond character of the C to 0 bond and causes absorption at longer
wavelength (shift absorption towards lower frequency).
The herbicides
bromacil, diuron, and chlorotoluron contain carbonyl, secondary and
tertiary amine and amide groups which enable them to participate with
HA in hydrogen bond formation and charge-transfer complexes.
The
1
appearance of strong absorption bands near 1650 cm , and the reduced
absorption of the 1717 cm
-1
band especially for diuron and chlorotol-
uron, provide clear indication for the formation of H- bonding and
probably charge-transfer complexes between these herbicides and HA.
The slight reduction of the absorption band near 1200 cm-1 particularly for diuron support the claim for H-bond formation.
Similar results
were reported for the substituted urea herbicides linuron, monuron,
monolinuron, metobromuron, cycluron and fenuron (Senesi and Testini,
1980 and 1983).
In the herbicide-HA spectra, particularly those for chlorotoluron
and bromacil, a number of unchanged absorption bands appeared in both
the IR spectra of pure herbicides (Fig. 13) and the IR spectra of
herbicide-HA complexes.
The presence of these bands made the task of
interpreting the spectra difficult because of the uncertainty of
whether these bands emerged as a result of the interactions or were
inherited from the parent herbicide molecules.
The existence of these
bands, on the other hand, strengthens the possibility that only weak,
physical-type interactions were involved in the binding of these
herbicides (bromacil, diuron and chlorotoluron) by HA (Senesi and
Testini, 1980).
It is interesting to note that although greatly
reduced in intensity, the 1717 cm-1 band of HA has persisted in all
61
herbicide-HA spectra.
This indicates that in these complexes a
considerable proportion of carboxylic acid (COOH) groups in humic acid
remained inaccessible to the herbicide molecules.
62
ACKNOWLEDGEMENT
The authors wish to acknowledge the contribution of Drs. S. C.
Fang and J. Baham.
The senior author wishes to express his apprec-
iation to the Libyan people and the University of Elfateh for
financing his graduate studies.
The organization and analysis of
the data base associated with this investigation were carried out in
part with the use of the PROPHET system, a unique national resource
sponsored by the Division of Research Resources, NIH, Bethesda,
Maryland 20014.
63
Table 6.
Elution parameter (Kav) for the herbicide and herbicide-HA
peaks and troughs determined on Sephadex G-25 gel.
Herbicide
Herbicide alone
Herbicide-HA peak
Herbicide trough
Elution parameter, Kava
Diuron
2.68
0.07
2.58
Chlorotoluron
2.65
0.06
2.67
Simazine
2.00
0.07
2.05
Bromacil
1.36
0.09
1.27
Glyphosate
0.72
0.07
0.72
Diquat
0.54
0.03
0.56
a
K
av
and V
V -V
e
o
17.--/r-, where Ve is the elution volume; V is the void volume;
o
"t '0
t
is the total volume of the column.
0.132
0.126
\\
.116
I
/
4-)
4/
01
APO
0
11).
00
oi
./\
I
I
,
4
to
cr
-r-
ca
0.106
0.096
I
0
100
2b0
2.0
Elution Volume (mL)
Figure 9a.
Elution profile for the measurement of diquat binding by
humic acid on Sephadex G-25 gel.
0.38
0.126
0.30
0.116
\
=
Cr
1"/
-/
I.
7 !I's
1
0
..
fit
0
Figure 9b.
'0
'
0 ' 0 (0°
/947\
0.20
0\
I
414k
1
O
O
O
0
1
0 .
0.106
0.096
4
0 0 /! ! -..
0. ,, ,.
f
100
0.10
II...
4....
4. 0...
I
200
'
-A
0.00
1
250
Elution profile of diquat-HA binding (.) superimposed on the elution
profile of humic acid (o; right-Y-axis) on Sephadex G-25 gel.
1.04
1.22
1.12
0.96
A
0
Z
I
'rave
=
fl: 1/416 Illi 11 I I III
111
k
4\
I\
Ili,'
i
INA
171.
0
r
A.
A
A-Atit f
A-A
1.02
\
il
0.86
A
_
=
:
0.92
tot
A
A.
0.76
0
A
A
1
100
A-A
.
1
200
V
_i.
.
300
I
400
A
.
0.82
500
Elution Volume (lt)
Figure 10.
Elution profile of diuron-HA binding () superimposed on elution curve
of diuron (A; right-Y-axis) on Sephadex G-25 gel.
\ .1\4
0.082
t
0
t
100
t
1
200
1
300
400
Elution Volume (mL)
Figure 11.
Elution profile for the measurement of simazine binding by
humic acid on Sephadex G-25 gel.
68
t
3500
Figure 12.
2500
.
1600
.
1000
500
Wavenumber
Infrared spectra of (A) humic acid (HA), (B) HA-diquat,
(C) HA-bromacil, (D) HA-diuron, and (E) HA-chlorotoluron.
69
A
3500
2500
1600
1000
500
Wavenumber (cm-1)
Figure 13,
Infrared spectra of herbicides (A) diquat, (B) bromacil,
(C) diuron, and (D) chlorotoluron.
70
LITERATURE CITED
1.
Bellamy, L. J. 1975.
The infrared spectra of complex molecules.
Chapman and Hall, London.
2.
Burchill, S., M. H. B. Hayes, and D. J. Greenland.
tion.
p. 221-400.
Adsorp-
In D. J. Greenland and M. H. B. Hayes (ed.) The
chemistry of soil processes.
3.
1981.
John Wiley & Sons, New York.
Burns, I. G., M. H. B. Hayes, and M. Stacey.
1973a.
Spectroscopic
studies on the mechanisms of adsorption of paraquat by humic acid
and model compounds.
4.
Pestic. Sci. 4:201-209.
Burns, I. G., M. H. B. Hayes, and M. Stacey.
1973b.
Studies of the
adsorption of paraquat on soluble humic fractions by gel filtration
and ultra filtration techniques.
5.
6.
Pestic. Sci. 4:629-641.
Fairclough, G. F., Jr., and J. S. Fruton.
1966.
interaction as studied by gel filtration.
Biochemistry 5:673-683.
Gaillardon, P., R. Calvet, and J. C. Gaudry.
of certain phenyl ureas by humic acids.
7.
Gilmour, J. T., and N. T. Coleman.
studies:
8.
Ca-H-humic acid.
Peptide-protein
1980.
The adsorption
Weed Res. 20:201-204.
1971.
s-Triazine adsorption
Soil Sci. Soc. Am. Proc. 35:256-259.
Grice, R. E., and M. H. B. Hayes.
1970.
Methods for studying the
adsorption of organic chemicals by soil organic matter preparations.
Proc. 10th Br. Weed Control Conf. 3:1089-1100.
9.
Hayes, M. H. B.
1970.
Adsorption of triazine herbicides on soil
organic matter, including a short review on soil organic matter
chemistry.
10.
Residue Rev. 32:131-174.
Hummel J. P., and W. J. Dreyer.
1962.
binding phenomena by gel filtration.
63:530-532.
Measurement of proteinBiochem. Biophys. Acta.
71
11.
Khan, S. U.
1973.
Interaction of humic substances with
bipyridylium herbicides.
12.
Khan, S. U.
acid.
13.
1974.
Can. J. Soil. Sci. 53:199-204.
Adsorption of bipyridylium herbicides by humic
J. Environ. Qual. 3:202-206.
Khan, S. U.
1977.
Adsorption of dyfonate (o-ethyl-s-phenyl
ethylphosphonodithioate) on humic acid.
14.
Khan, S. U.
1978.
The interaction of organic matter with
pesticides. p. 137-171.
Organic Matter.
Can. J. Soil Sci. 57:9-13.
In M. Schnitzer and S. U. Khan (ed.) Soil
Elsevier Scientific Publishing Company, New York,
NY.
15.
Khan, S. U., and R. Mazurkewich.
humic acid.
16.
1974.
Adsorption of linuron on
Soil Sci. 118:339-343.
Malcolm, R. L.
Method and importance of obtaining humic
1976.
and fulvic acids of high purity.
J. Res. U. S. Geol. Survey
4:37-40.
17.
Mueller-Wegener, U.
1977.
Binding of s-triazines to humic acids.
Geoderma 19:227-235.
18.
Nearpass, D. C.
humin.
19.
1976.
Adsorption of picloram by humic acids and
Soil Sci. 121:272-277.
Senesi, N., and C. Testini.
1980.
herbicides by soil humic acids.
20.
Senesi, N., and C. Testini.
Adsorption of some nitrogenated
Soil Sci. 130-314-320.
1982.
Physico-chemical investigations
of interaction mechanisms between s-triazine herbicides and soil
humic acid.
21.
Geoderma 28:129-146.
Senesi, N., and C. Testini.
1983.
Spectroscopic investigation of
electron donor-acceptor processes involving organic free radicals
in the adsorption of substituted urea herbicides by humic acids.
72
Pestic. Sci. 14:79-89.
22.
Silverstein, R. M., G. C. Bassler, and T. C. Morrill.
Spectrometric identification of organic compounds.
1981.
John Wiley &
Sons, New York.
23.
Stevenson, F. J.
1976.
Pesticides in Soil.
S. K.
Organic Matter Reactions Involving
In D. D. Kaufman, G. G. Still, G. Paulson, and
Bandal (ed.) Bound and conjugated pesticide residues.
ACS
Symp. Ser. 29:180-207.
24.
Stevenson, F. J.
1982.
Humus chemistry.
John Wiley & Sons, New
York.
25.
Sullivan, J. D., Jr., and G. T. Felbeck, Jr.
1968.
A study of
the interaction of s-triazine herbicides with humic acids from
three different soils.
26.
Soil Sci. 106:42-52.
Wood, G. C. and P. F. Cooper.
1970.
The application of gel
filtration to the study of protein-binding of small molecules.
Chromatogr. Rev. 12:88-107.
73
CHAPTER III
Association of Selected Herbicides With Water Soluble
Organic Materials from Soil
1
Y. A. El-Madhun, J. L. Young and V. H. Freed2
1
Contribution of the Departments of Agricultural Chemistry and Soil
Science, Oregon State University, Corvallis, Oregon
2
97331.
Research Assistant, Research Chemist ARS-USDA and Professor of Soil
Science, and Professor of Chemistry, respectively.
Any use of trade names is for descriptive purposes only and does not
imply endorsement by OAES/OSU or ARS-USDA.
74
ABSTRACT
The binding of six herbicides:
diquat (1,1-ethylene-2,2-bipyri-
dylium ion), glyphosate [N-(phosphonomethyl) glycine], simazine
(2-chloro-4,6-bis-ethylamino-s-triazine), bromacil (5- bromo- 3 -sec-
butyl-6-methyluracil), diuron [3-(3,4-dichloropheny1)-1,1-dimethylurea], and chlorotoluron [3-(3-chloro-4-methylpheny1)-1,1-dimethylurea] by organic materials extracted with distilled water from an
organic soil was studied by gel filtration chromatography.
The gel
filtration method revealed that most constituents of the water soluble
soil organic materials (WSSOM) had molecular weights between 5000 and
700 daltons.
Chromatographic procedures, as adapted by Hummel and
Dreyer (1962) to detect interactions between small organic compounds
and macromolecules, were very successful in proving the existence of
binding between the herbicides and WSSOM.
Observed differences in the
elution curves reflected the characteristics of the interacting
herbicides as well as their associations with WSSOM.
The nonionic
herbicides bromacil, diuron, chlorotoluron, and simazine showed
comparable affinity for the three main fractions of WSSOM, whereas the
herbicide glyphosate showed stronger tendency to complex with the most
retarded fraction of WSSOM.
The elution data reinforced the importance
of adhering to the method requirements.
Changes in infrared (IR) band
position and intensity, particularly near 1640 cm-1 in the herbicidesWSSOM IR spectra, provided strong evidence for the involvement of an
ion exchange mechanism in diquat and of hydrogen bonding in bromacil,
diuron, and chlorotoluron interactions with WSSOM.
Additionally
observed shifts in some characteristic IR absorption bands suggested
the formation of charge transfer complexes between bromacil, diuron,
75
and chlorotoluron and WSSOM but not between diquat and water soluble
soil organic materials.
These results appear significant in assessing
herbicides mobility and transport in soil and water, especially when
water pollution is a matter of concern.
Additional Index Words:
gel filtration, infrared spectroscopy,
bromacil, diuron, chlorotoluron, diquat, glyphosate, simazine.
76
INTRODUCTION
Soil organic materials have been the subject of considerable
interest in recent years because of their ability to interact with
pesticides in soil and water.
Compounds that reportedly interact with
humic substances include s-triazines (Khan, 1978; MUller-Wegener, 1977;
Gilmour and Coleman, 1971; Sullivan and Felbeck, 1968; Senesi and
Testini, 1980 and 1982); bipyridilium ions (Burns et al., 1973a, 1973b;
Khan, 1973, 1974); substituted ureas (Gaillardon et al., 1980; Khan
and Mazurkewich, 1974; Senesi and Testni, 1980, 1983); and chlorinated
hydrocarbons (Ballard, 1971; Choi and Chen, 1976; Pierce et al., 1974).
According to Ballard (1971), the downward movement of the insecticide
DDT in a forest soil is caused by water-soluble, humic-like substances.
Ogner and Schnitzer (1970) reported that fulvic acid, a water-soluble
humic component with relatively low molecular weight which occurs
widely in soils and waters, can combine with lyophobic organic
compounds such as alkanes and fatty acids to form stable water-soluble
complexes.
Pierce et al. (1974) suggested that suspended humic
particulates in aqueous solutions may serve as transporting agents for
chlorinated hydrocarbons through the water column and for accumulating
them in sediments.
In view of the foregoing, water-soluble soil organic substances
may have special functions with regard to herbicide mobilization and
transformation, i.e. they may enhance the solubility of certain
pesticides (Wershaw et al., 1969), may act as vehicles for the
transport of pesticides in soils and natural water, and may function
also in mediating and promoting the nonbiological degradation of
77
many pesticides (Khan, 1978; Stevenson, 1982).
Yet, precise charac-
terization of the interactions occurring between humic substances
particularly the water-soluble components and pesticides has remained
quite difficult.
Various methods have been used to study the interaction of pesticides with soil and with isolated humic substances.
The most common
approach has been the slurry method which involves the bringing together of known amounts of solid and herbicide solution; then after
equilibration, the solution concentration is measured and the decrease
in the herbicide concentration is determined.
This technique is not
suitable in evaluating herbicide binding by soil organic substances,
especially the water soluble components (Hayes, 1970), because it
makes no distinction between the herbicide molecules that exist free
in soil solution and those bound to water-soluble soil organic
materials.
A gel chromatography technique based on the procedures of Hummel
and Dreyer (1962) provides an appropriate alternative for determining
the binding of small organic chemicals such as pesticides by watersoluble humic polymers.
The procedure has been used by a few workers
to study the binding of herbicides (diquat and paraquat) by humic
substances (Burns et al., 1973b; Khan, 1973).
More recently we
utilized the Hummel and Dreyer method to study the binding of several
ionic and nonionic herbicides by a soil humic acid (Chapter II).
Among the several spectrometric techniques that can be used to
characterize the nature of the binding mechanism(s), infrared spectroscopy has been the most commonly chosen.
This technique has shown the
involvement of an ion exchange process in diquat, paraquat, chlordi-
78
meform and triazines (Burns et al., 1973a; Khan, 1973, 1974; Maqueda
et al., 1983; Senesi and Testini, 1980; Sullivan and Felbeck, 1969);
hydrogen bonding in triazines and substituted ureas (Senesi and
Testini, 1980, 1983); and charge transfer complexes in triazines,
substituted ureas, and bipyridilium herbicides binding by soil humic
substances (Khan, 1973, 1974; Willer-Wegener, 1977; Senesi and
Testini, 1980, 1982, 1983).
We have obtained evidence for ion
exchange reactions in diquat and H-bonding in bromacil, diuron and
chlorotoluron association with humic acid (Chapter II).
The objectives of this study were:
(i) to assess the inter-
actions between herbicides and the water soluble organic materials
extracted from soil; (ii) to establish the applicability of the
Hummel and Dreyer gel filtration procedure as an alternative method
in binding studies involving water soluble soil organic compounds;
and (iii) to characterize the binding mechanisms the herbicides
undergo when in association with dissolved organic substances
utilizing infrared spectroscopy.
This work is a parallel invest-
igation to studies on reactions between these herbicides and larger
molecular weight humic substances extracted from soil.
79
MATERIALS AND METHODS
Water soluble soil organic materials (WSSOM) were extracted from
Semiahmoo mucky peat soil.
The soil sample was collected from the
surface layer (0-20 cm) from a farm near Salem, Oregon.
The soil was
chosen because of its high organic carbon content of 27.8%.
The
organic carbon content was determined from the weight loss upon
ignition multiplied by 0.58.
WSSOM was extracted by mixing a known
quantity of soil in a measured volume of distilled water in Corex
glass centrifuge tubes to give 1:5 soil to water ratio, and equilibrating on a mechanical shaker for 24 h at 25° C.
fugation at 1085 RCF for 1
Following centri-
h, the yellow clear supernatant was
removed and filtered through Carl Schleicher and Schuell filter paper
No. 597.
The carbon analysis was performed on an Oceanography Inter-
national total carbon analyzer, model 0524, equipped with Lira IR
detector model 303.
The carbon content of WSSOM was 238 mg L
The pH of this extract was 5.50.
-1
.
WSSOM was usually used immediately
after preparation or stored at 3° C with no chemical additives.
WSSOM
was discarded if not used within 24 h from the time of preparation.
The herbicides selected for the study included the ionic compounds
diquat and glyphosate, and the nonionic chemicals bromacil, diuron,
chlorotoluron and simazine.
These compounds were chosen because they
represent major herbicide groups and are widely used on agricultural
crops.
The herbicides were 14C-labelled and were >99% analytically
and radiochemically pure.
The gel filtration procedure of Hummel and Dreyer (1962) was
employed to descry any interaction between herbicide molecules and
80
water soluble organic materials.
This chromatographic method, which
is analogous in principle to dialysis equilibrium, was devised to
detect reversible interactions between macromolecules and substances of
low molecular weight.
It avoids the difficulties encountered in zonal
gel filtration approach due to binding of ionic and aromatic compounds
with the Sephadex gel.
It also eliminates the problem of dissociation
of macromolecule - ligand complex in the chromatographic column.
The gel filtration procedure was carried out by packing a 26 x
370 mm LKB 2137 chromatographic column with fine mesh, Sephadex G-25
gel (exclusion limit 5000).
The column was equilibrated with phos-
phate-buffer solution of pH 6.85 and ionic strength 0.05M KC1.
Dextran 2000, at 2 g L
-1
Blue
concentration was used to determine the
uniformity of packing and the void volume of the column.
The elution profile of the water-soluble organic materials was
determined by applying 20 mL of WSSOM to the column and eluting with
the buffer at 25° C in the descending direction using a peristaltic
pump driven by a motor at a flow rate of 60 mL h
-1
.
The emerging
eluate was collected in 5 mL portions on a fraction collector (ISCO,
model 328).
The WSSOM concentration in each fraction was assessed by
measuring the absorbance at 400 and 280 nm on a Beckman, model 25,
UV-Visible Spectrophotometer.
Interactions between water-soluble organic materials and herbi-
cides were determined by adding 20 mL of the following pM concentrations of WSSOM-herbicide solutions to the column:
bromacil, 0.966;
chlorotoluron, 0.311; diuron, 1.01; diquat, 0.125; glyphosate, 1.01;
and simazine, 0.109.
The chromatographic column was eluted with the
phosphate buffer which had an identical herbicide concentration as in
81
the WSSOM-herbicide solution introduced to the column.
The column
effluent was continuously monitored by ISCO model UA-2 dual UV
analyzer set at 280 nm wavelength to follow the elution behavior of
WSSOM-herbicide complex.
The herbicide concentration in each fraction
was determined by taking 0.5 mL aliquots and measuring the radioactivity in solution with Packard model 3330 Tri-Carb Liquid Scintillation Spectrometer after adding 3.5 mL of Handifluor counting fluid.
An internal standard was used to evaluate the instrument counting
efficiency regularly throughout the study period.
Infrared (IR) analysis was performed on a Perkin-Elmer model 457
grating infrared spectrophotometer in the 4000 to 250 cm-1 wavenumber
range in KBr solvent matrix.
Herbicide solutions of 1 mM concentration
were prepared by dissolving 7.2 mg diquat, 5.2 mg bromacil, 4.7 mg
diuron, and 4.2 mg chiorotoluron in 20 mL WSSOM in Corex centrifuge
tubes.
The tubes were securely capped and shaken on a mechanical
shaker for 15 h before subjected to freeze-drying to remove the excess
moisture.
The pellets were prepared by mixing 250 mg KBr with 1 mg of
the freeze-dried sample.
following:
The IR recording conditions were the
reference, air; cell path, 1 mm; slit, normal; and scan
time, medium.
A polystyrene film with 0.05 mm cell path was regularly
used to calibrate absorption band positions.
82
RESULTS AND DISCUSSION
Gel Filtration
The elution curves of the straw-colored, water-soluble soil
organic materials determined on Sephadex G-25 and G-10 gels are shown
in Fig. 14.
The elution curve obtained on Sephadex G-25 shows a
plateau with three peaks.
Based on this elution behavior WSSOM may be
divided into three main fractions with regard to molecular weight.
Fraction 1, whose molecular weight is
5000, was eluted with the void
volume, whereas, the fraction whose components have intermediate
molecular size (fraction 2) emerged in the middle of the elution curve.
Fraction 3, which produced the strongest peak in the elution curve,
emerged last from the column and corresponds to the fraction with the
smallest molecular weight.
Since, on Sephadex G-10, whose exclusion
limit is 700, WSSOM eluted in a single sharp peak at the void volume
with little tailing (Fig. 14), the molecular size of the majority of
WSSOM components is most likely between 700 and 5000 daltons.
Humic substances generally comprise one-third to one-half of the
dissolved organic carbon in surface waters (Malcolm, et al., 1981;
Martin and Reuter, 1973; Thurman, et al., 1982).
The molecular size
of dissolved organic materials in water varies, due to such factors as
sourceof material, method of determination, and aggregation of humic
substances in solution (Thurman et al., 1982).
Most values, however,
are in the molecular weight range of 500 to 10000 daltons (Martin and
Reuter, 1973; Oliver and Thurman, 1983; Thurman et al., 1982).
The
reported molecular weight range of aquatic fulvic acids is 500 to 2000
daltons (Malcolm et al., 1981; Thurman and Malcolm, 1983; Thurman
83
et al., 1982).
Other researchers have shown that fulvic acids from
soil also are in that same range (Ogner and Schnitzer, 1970).
The
aquatic organic substances resemble soil fulvic acids by being watersoluble and of relatively low molecular weight (Baham and Sposito,
1983; Beck et al., 1974).
The elution behavior of bromacil-WSSOM, given in Fig. 15a, shows
an ideal performance of the gel filtration method.
The three distinct
peaks coincided nicely with WSSOM peaks indicating that the three main
fractions of WSSOM complexed with the herbicide.
Fig. 15a also shows
that more bromacil complexed with fraction three of WSSOM (low MW
fraction) than with other components of WSSOM.
In Fig. 15b, the
elution curve of bromacil is shown superimposed on bromacil-WSSOM
elution curve.
The emergence of the herbicide-WSSOM trough at the
elution volume of bromacil should be noted since it fulfills a
criterion for the binding of the herbicide with WSSOM (Hummel and
Dreyer, 1962).
The elution diagrams of the remaining nonionic herbicides diuron,
chlorotoluron, and simazine complexes with the water-soluble soil
organic substances are shown in Figs. 16, 17, and 18.
Common features
in the three curves are the multiple peaks coinciding with WSSOM peaks
and the appearance of herbicide-WSSOM complex troughs at the same
elution volumes as the herbicides.
As in bromacil, these herbicides
appear to form complexes with all three major components of WSSOM and
the affinity of the herbicides for the three fractions seem very
comparable.
These patterns provide further evidence for the existence
of interactions between herbicides and WSSOM and corroborate the
reproducibility of the experiments as well as the adequacy of the
84
procedure.
The elution curves of the herbicides glyphosate and diquat shown
in Figs. 19 and 20, look rather different from the nonionic herbicides
discussed above.
Glyphosate-WSSOM complex shows a single sharp peak
coincidental with fraction 3 peak of WSSOM and a trough appeared near
the elution volume of glyphosate.
This elution behavior suggests that
glyphosate has a very high affinity for the most retarded (lowest MW)
fraction of WSSOM.
However, this strong tendency to associate with
fraction 3 does not mean the lack of affinity for the other constituents of WSSOM since earlier results show that glyphosate interacts and
forms complexes with humic acids from soil.
The apparent lack of
interaction between glyphosate and the relatively large-sized WSSOM
components could be due to the low concentration of glyphosate.
The
sharp drop from the peak maxima to the trough minima in the elution
curve (Fig. 19) was necessary since the elution volume of glyphosate is
near where the trough has emerged.
Diquat-WSSOM elution diagram (Fig. 20) is different and shows nonideal behavior.
The same pattern was obtained each time the elution
experiment was repeated.
Fig. 20a shows that WSSOM fractions 1 and 2
interacted with the herbicide as indicated by the coincidence of the
peaks.
The disappearance of a peak corresponding to fraction 3 complex
with diquat was due to its occurrence in the herbicide zone (Fig. 20b)
where the strong and sharp trough appeared.
The presence of a second
broad but rather sharp trough in the elution curve may be due to the
occurrence of a second diquat deficient zone resulting from the
complications brought about by the overlapping of fraction 3 plus
diquat complex peak and the herbicide trough.
Hence, this strange
85
looking pattern is most likely due to the coincidence of
fraction 3
complex peak with the herbicide trough and it may also be related to
the existence of interaction between the gel matrix and diquat and the
gel and WSSOM components, particularly those with low molecular weight.
One of the principal requirements for a satisfactory performance of the
gel filtration method is the separation of the complex peak from the
trough by a distance in which the herbicide concentration returns to
its original level (Fairclough and Fruton, 1966; Hummel and Dreyer,
1962; Wood and Cooper, 1970).
Obviously this demand was not met in
glyphosate or diquat association with WSSOM.
The relevance of the
Hummel and Dreyer (1962) method for studying the interaction of these
two herbicides with humic materials from soil has been documented in an
earlier study by these authors (Chapter II).
The potential role of WSSOM in pesticides mobility and transport
in the soil and water environments has long been suggested (Ballard,
1971; Ogner and Schnitzer, 1970; Pierce et al., 1974; Wershaw et al.,
1969) but little examined for lack of an adequate procedure.
In this
study, data have been presented in favor of the chromatographic method
developed by Hummel and Dreyer (1962).
The procedure is well suited
and probably is the method of choice for the study of ionic and
nonionic pesticide interactions with WSSOM as it was with soil humic
substances when the method is adequately applied and its requisites
are fulfilled.
Infrared Spectroscopy
Infrared spectra of untreated WSSOM, herbicides, and WSSOM-herbicide complexes were recorded to characterize the binding mechanisms
86
occurring in WSSOM-herbicide complexes.
The IR spectrum of the
untreated WSSOM preparation displays broad and poorly resolved
absorption bands reflecting the presence of many kinds of functional
groups in differing chemical environments.
Although the infrared
data provide a wealth of information about the sample, many of the
absorption bands can not be interpreted unequivocally (Silverstein
et al., 1981; Dyer, 1965).
The infrared spectrum of WSSOM reproduced as pattern A in Fig.
21 shows absorption bands in the range 3700-3000 cm
-1
(centered at
3400), assigned to intermolecular H-bonded 0 -H groups, and N-H and
aromatic C-H stretching vibrations; near 2940 cm
- 1
, due to aliphatic
C-H stretching; in the region 1800-1500 cm-1, attributed to (i) C=0
stretching, (ii) aromatic C=C stretching, (iii) H-bonded C-0 groups of
quinones and ketones, (iv) C00- symmetric stretching, (v) N-H bending
vibrations; near 1390 cm -1, due to C-H and 0 -H bending, C-0 stretching
of alcohols and phenols, and C00- asymmetric stretching; and in the
1200-1000 cm-1 region, assigned to 0-H deformation and C-0 stretching
of COOH (Stevenson, 1982; Silverstein et al., 1981; Dyer, 1965).
The spectrum of WSSOM is similar to the spectra of humic and
fulvic acids reported by Senesi and Testini (1980), Stevenson and Goh
(1971), Sullivan and Felbeck (1968).
Further, the spectrum closely
resembles the IR pattern published by Baham and Sposito (1983) for the
water-soluble organic fraction of Rialto sewage sludge.
These authors
demonstrated the occurrence of polypeptides, amino sugars, aliphatic
acids and carbohydrates in their preparation, suggesting the presence
of these substances in addition to humic materials in WSSOM.
The most significant differences between diquat-WSSOM spectrum
87
(Fig. 21) and the untreated WSSOM spectrum is the increased intensity
of the broad band centered at 1640 cm-1
.
As mentioned above, this
absorption band is attributable to a number of functional
of which is the carbonyl group of carboxylic acid (COOH).
groups, one
The presence
of a rather weak shoulder at 1720 cm-1 provides further indication for
the occurrence of free carboxylic acid in the WSSOM preparation (Dyer,
1965).
The appearance of a fairly sharp band in the 1615 cm-1 region
with the diminishing of the weak shoulder at 1720 cm-1 provides
indication for the conversion of COOH to C00- upon the addition of the
herbicide suggesting the involvement of an ion exchange mechanism in
diquat binding with WSSOM.
These results are entirely consistent with
those discussed in other sections and with data other researchers
reported for diquat interactions with humic and fulvic acids (Burns et
al., 1973a; Khan, 1974).
In this study, indication was found in the diquat-WSSOM spectrum
for the formation of charge transfer complexes, suggesting that either
this mechanism is not involved or that the absorption bands corresponding to these complexes were not well resolved to permit positive
identification.
Khan (1973, 1974) presented data for the formation of
charge transfer complexes between diquat and the humic compounds;
however, his results could not be confirmed by these workers nor by
others (Burns, 1973a).
The occurrence of a charge transfer mechanism
in diquat-WSSOM seems highly possible since humic materials have been
shown to possess charge transfer donor and acceptor properties
(Lindqvist, 1982, 1983).
The IR spectra for the herbicides bromacil, diuron, and chiorotoluron interaction products with WSSOM (C and D Fig. 21) display
88
several significant divergences from the spectra of the pure herbicides
and untreated WSSOM.
Like diquat-WSSOM spectrum, these spectra show
changes in the broad absorption band centered near 1640 cm-1 (overlapping of C=0 stretching, amide I band, and N-H bending, amide II
band).
Unlike diquat, however, these herbicides are nonionic and very
weakly basic.
Therefore, the observed sharpening and increase in
intensity of the 1640 cm
-1
band, particularly in diuron and bromacil,
can not be explained by the conversion of COOH to C00
(Stevenson, 1972; Senesi and Testini, 1980, 1983).
as in diquat
These effects are
most likely due to the shifting towards lower wavenumbers of the 0 -H
and N-H bending and C-0 stretching frequencies of the N-H, 0 -H and C-0
groups of WSSOM when involved in hydrogen bonding with the herbicides
(Silverstein et al., 1981).
Depending on the strength of the
H-bonding, the frequency reduction ranges from less than 15 cm
cm
-1
-1
to 50
or more (Silverstein et al., 1981).
Hydrogen bonding can occur in any system that contains a proton
donor group such as 0 -H and N-H, and a proton acceptor like oxygen,
nitrogen, and halogens.
The herbicides diuron, chlorotoluron and
bromacil possess such chemical moieties as the carbonyl and amide
groups which offer strong possibility for them to undergo H-bonding
with WSSOM (Stevenson, 1982).
The IR data, therefore, confirmed the
kind of bonding expected from knowledge of the chemical properties of
the herbicides.
Other changes in the IR spectra of the herbicides which support
the formation of H-bonding include:
the increased sharpness, espec-
ially in bromacil and chlorotoluron, of the absorption band near 1384
cm
-1
,
attributed to 0 -H bending and C-0 stretching vibrations; and the
28
observed alteration of bands particularly in chlorotoluron in the range
1200-1040 cm
-1
, arising from C-0 stretching and 0-H bending.
Hydrogen
bond formation has been one of the suggested (Hayes, 1970; Stevenson,
1972; Weed and Weber, 1974) and experimentally demonstrated mechanisms
for the binding of several triazine and substituted urea herbicides
(Senesi and Testini, 1980, 1983; Sullivan and Felbeck, 1968).
Careful study of the nonionic herbicides-WSSOM spectra reveal
some very interesting observations.
The band at 1420 cm-1 in bromacil
has been shifted to a new position (1384 cm-1) and the 738 cm-1 band
apparently moved to lower frequency (730 cm-1), whereas the absorption
band at 840 cm
-1
has either disappeared or overlapped with the band at
770 cm-1 in bromacil-WSSOM.
Similar shifts were observed in diuron-
WSSOM absorption vibrations also; the 870 cm-1 shifted to 863 cm-1 and
the 820 cm-1 appeared at 810 cm-1 in diuron-WSSOM complex spectrum.
These shifts may be due to changes in the out-of-plane C-H vibration
frequencies, suggesting the formation of charge transfer complexes
between the herbicides and WSSOM components.
Experimental evidence for
the formation of charge-transfer complexes between a number of
triazine and substituted urea herbicides and other chemicals has been
reported (Maqueda et al., 1983; Senesi and Testini, 1980, 1982, 1983).
90
ACKNOWLEDGEMENT
The authors wish to acknowledge the contribution of Drs. S. C.
Fang and J. Baham.
Sincere appreciation is due to Drs. A. P. Appleby
and I. J. Tinsley for their review of the manuscript.
The senior
author wishes to express his appreciation to the Libyan people and the
University of Elfateh for financing his graduate studies.
The
organization and analysis of the data base associated with this
investigation were carried out in part with the use of the PROPHET
system, a unique national resource sponsored by the Division of
Research Resources, NIH, Bethesda, Maryland 20014.
-0.18
0.5
/
111
Joici
0.4
0.12
0.3
0.2
.70.06
0.1
0.0 MOMMIMMOMMI:
0.0
100
200
300
0.00
Elution Volume (mL)
Figure 14.
Elution behavior of water soluble soil organic materials (WSSOM)
determined on Sephadex G-25 () and G-10 (, right-Y-axis) gels.
a
0.16
1.22
0.12
N7
i
I Wye,
_iVv \
V vi
V N7'
N7
1,,,v
vt.7
Vr.,V
V\ k..2___' /v
IwItv47 I Vvyv
V \/N7
v vci,
.
0.08
\I:1;r'
V
\
1.02
Is
I
VI;:7
y
V
V
I
0.92
i
IL
JL
i
0.04
I
l
1
.
.
0.00
'
0
100
200
300
380
Elution Volume (mL)
Figure 15a.
Elution profile of WSSOM-bromacil (v) superimposed on elution
curve
of WSSOM (11, right-Y-axis) determined on Sephadex G-25 gel.
1,28
2.0
1.22_
N7
'A?
V Y
v; V V
NYW W
rt,
1 .
1A771"7V7
N7
V V N7,7
V
1 2
AvA,
V IIV
N7
V
v
v°7
1.0
vv
1.02
/
_AAA&
V
0.92
0
100
200
t i
300
0.0
380
Elution Volume (ml)
Figure 15b.
Elution profile of WSSOM-bromacil (v) superimposed on elution curve of
bromacil (A, right-Y-axis) determined on Sephadex G-25 gel.
0.097
C)
0.16
0.093
ONO
O
0.12
O
0.089
0)00
0.085
si)10
_C&I
11 109501 10. 0.08
0
0.04
0.081
0.077
I
I
.
.
I
0
100
200
.
I
300
0.00
.
400
Elution Volume (mL)
Figure 16.
Elution profile of WSSOM-Simazine (0) superimposed on elution curve
of WSSOM (, right-Y-axis) determined on Sephadex G-25 gel.
0.84
0
100
200
300
0.00
400
500
Elution Volume (mL)
Figure 17.
Elution profile of WSSOM-diuron
(o) superimposed on elution curve
(, right-Y-axis) determined on Sephadex
of WSSOM
G-25 gel.
0.16
0.34 A
.10
0.12
21
a 0.32
O
=
7,
0
.
2
s-
-.0.08
41A1
0.30
-
ist,
I4
A A AN4t,
1 °Lk
.40?x6A'Al
AA
'
A
II
I d'A' 4041
AA
0.28
0.27
t
A
p
0.00
1
100
0.04
200
300
400
500
Elution Volume (mL)
Figure 18.
Elution profile of WSSOM-chlorotoluron (&) superimposed on elution curve
of WSSOM (, right-Y-axis) determined on Sephadex G-25 gel.
2.10
0.16
I
1.70
0.12
1.30
0.08
0
absVcAssIsz*D9ziv
0.90
0.04
O
oo
0.50
.
.
0.00
'
0
200
300
380
Elution Volume (mL)
Figure 19.
Elution profile of WSSOM-glyphosate (0) superimposed on elution curve
of WSSOM (II, right-Y-axis) determined on Sephadex G-25 gel.
1
cm
o
0.12
.
/
0.16
9.
I
ell
II
loll
1111,, :
1
:E.I
4
- 0.12
0.08 _
1111141
,
=
I
0
_
0-
9cm
..
[lb
-
tr-T1
0
Po_
0
_ 0.08
1:1
\
0.04
--
01
I
0.04
\
0 0
-
I
451
.
0.00
'
0
100
*
1
t
200
300
0.00
'
380
Elution Volume (mL)
Figure 20a.
Elution profile of WSSOM-diquat (o) superimposed on elution curve of
WSSOM (U, right-Y-axis) determined on Sephadex G-25 gel.
7.20
6.40
i 0.08
Illh
0 eork
4.,
=
v
0
0
or
C3
I
d 1
0.04
i
\0
OD
1
"POI
ill
1
5.40
0
03
40)
1
a
/ \ /
.
0.00
0
40
_
f
11111
100
1
11
1"i
i
1
I
C]
1
......II
A
200
.
4, .
1
300
I
4.40
380
Elution Volume (mL)
Figure 20b.
Elution profile of WSSOM-diquat (0) superimposed on elution curve of
diquat (, right-Y-axis) determined on Sephadex G-25 gel.
100
A
3500
2500
1600
1000
Wavenumber (cm-1)
Figure 21.
Infrared spectra of (A) WSSOM, (B) WSSOM-diquat, (C)
WSSOM-bromacil, (D) WSSOM-diuron, and (E) WSSOMchlorotoluron.
500
101
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Thurman, E. M., and Malcolm, R. L.
humic substances:
1983.
Structural study of
New approaches and methods.
pp. 1-23.
In
R.F. Christman, and E.T. Gjessing (ed.) Aquatic and Terrestrial
Humic Materials.
36.
Ann Arbor Science, Ann Arbor, Michigan.
Thurman, E. M., R. L. Wershaw, R. L. Malcolm, and D. J. Pickney.
1982.
Molecular size of aquatic humic substances.
4:27-35.
Org. Geochem.
105
37.
Wershaw, R. L., P. J. Burcar, and M. C. Goldberg.
action of pesticides with natural organic material.
1969.
Inter-
Environ.
Sci. Technol. 3:271-273.
38.
Wood, G. C. and P. F. Cooper.
1970.
The application of gel
filtration to the study of protein-binding of small molecules.
Chromatogr. Rev. 12:88-107.
106
CHAPTER IV
Degradation of Diuron, Chlorotoluron, and Bromacil
by Soils at Three Temperatures'
Y. A. El-Madhun, and V. H. Freed
1
Contribution of the Departments of Agricultural Chemistry and
Soil Science, Oregon State University, Corvallis, Oregon
2
97331.
Research Assistant and Professor of Chemistry, respectively.
107
ABSTRACT
The rate of diuron [3-(3,4-dichloropheny1)-1,1-dimethylurea],
chlorotoluron [3-(3-chloro-4-methylpheny1)-1,1-dimethylurea], and
bromacil (5- bromo -3- sec - butyl -6- methyluracil) degradation in two soils
was measured at 25, 30, and 35° C.
monitoring
14
Degradation was assessed by
CO2 evolution from herbicide-treated soils.
ation of the three herbicides was slow with no lag phase.
The degradDiuron and
chlorotoluron break down was higher in Adkins loamy sand than in
Semiahmoo mucky peat.
by soil type.
Bromacil degradation was essentially unaffected
The affect of temperature on degradation was more
pronounced than the effect of soil type or herbicide concentration.
The rate of
14
CO2 production was approximately tripled by a 10-degree
rise in temperature from 25 to 35° C.
evolution from soil only doubled.
However the rate of total CO2
The presence of herbicides in the
soil did not affect the activity of the microorganisms as measured by
the total CO
2
production, indicating that the degradation was a
co-metabolism process.
first-order rate law.
Degradation of the three compounds followed a
The estimated half-lives ranged from 223 to
4101 days for diuron, 288 to 3397 days for chlorotoluron and 2596 to
46200 days for bromacil.
Evaluation of the rate of degradation at
three temperatures permitted calculation of the energy of activation
from the Arrhenius equation.
Additional Index Words:
evolution, co-metabolism.
First-order kinetics, half-life,
14
CO2
108
INTRODUCTION
Knowledge of decomposition of herbicides in soil is important to
both agricultural management and environmental safety.
The main
processess leading to degradation of pesticides in soil are biological
action, photochemical reaction and chemical reaction either operating
independently or in combination.
It may not be reasonable to expect a
simple straight-forward behavior in degradation in so complex a milieu
as the soil (Freed and Haque, 1973).
In general, however, the dominant
route for the loss of many herbicides in soil is microbial metabolism
(Ashton, 1982; Freed and Haque, 1973).
Microbial involvement in decomposition of many pesticides
including the substituted urea herbicides - of which diuron and
chlorotoluron are examples - as well as bromacil has been shown by
reduced degradation in sterilized soil (Hill et al., 1955; Wolf and
Martin, 1974), evolution of 14CO2 from soil treated with 14C-labeled
compounds (Gardiner et al., 1969; Gerstl and Yaron, 1983; Lopez and
Kirckwood, 1974; McCormick and Holtbold, 1966); increased rate of loss
upon the addition of an energy source such as glucose (McCormick and
Holtbold, 1966; Murray et al., 1968; Wolf and Martin, 1974); and
degradation of herbicides in isolated cultures of microorganisms
(Attaway et al., 1982; Lopez and Kirckwood, 1974; Murray et al., 1968).
The rate of degradation of a herbicide in soil is influenced by
various factors.
Such factors include sorption, soil type, herbicide
concentration, temperature, and soil moisture content.
In general,
conditions that promote microbial activity enhance the rate of loss,
and those that inhibit the growth of microorganisms reduce the rate
(Ashton, 1982).
109
The effect of temperature and soil moisture on the rate of
degradation of herbicides has been studied by a number of investigators
(Gerstl and Yaron, 1983; Majka and Lavy, 1977; Walker, 1976).
Such
studies have, however, usually been restricted to one soil; there are
few cases in which the effect of soil type on degradation has been
examined under similar experimental conditions.
The effect of sorption
on herbicides degradation has been the subject of reviews by GrahamBryce, (1981), Hance, (1970), and Hurle and Walker, (1980).
Hill et al. (1955) proposed that degradation of the substituted
urea herbicides in soil followed approximately a first-order rate law
and presented limited data to show this for monuron and diuron.
Burschel and Freed (1959) reasoned that since in soil, microorganisms
are non-limiting or capable of becoming so and soil nutrients are
normally present in enormous quantities compared with the pesticides,
the rate-limiting component would be the herbicide concentration, hence
first-order kinetics should be observed.
Hance and Haynes (1981)
argued that this proposition applies only to compounds that do not
induce adaptation changes in the soil microbial population which seem
to be a majority of soil applied herbicides.
Actually very few kinetic
studies have been reported and most of these studies did fit the firstorder rate law permitting the calculation of the half-life and
activation energy of degradation (Walker and Roberts, 1978; Wolf and
Martin, 1974; Zimdahl et al. 1970).
However, soil is such a complex
biological and chemical medium that it is not surprising to find
deviations from simple first-order kinetics.
Such deviations have
been shown for the degradation of linuron and metribuzin herbicides in
soil (Hance and Haynes, 1981; Kempson-Jones and Hance, 1979).
Because
110
understanding the kinetics of herbicides degradation in soil is thus
far from complete, one objective of the work presented here was to
acquire more quantitative information on the kinetics of degradation
and to estimate the half-lives of diuron, chlorotoluron and bromacil
in soil.
In particular little prior information is available on the
fate of the herbicide chlorotoluron.
The other objective of the study
was to assess the influence of temperature and herbicide concentration
on the rate of degradation of the three herbicides.
111
MATERIALS AND METHODS
The soil samples used in the incubation study were collected from
the top 20 cm layer of the soil profile of two soil types in Oregon.
These soils were Adkins loamy sand (coarse loamy, mixed, mesic,
Xerollic Camborthid) and Semiahmoo mucky peat (Euic, mesic, Typic
Medisaprist).
mm mesh seive.
Samples were air-dried and ground to pass through a 2
Selected properties of the two soils are shown in
Table 1.
Carbonyl-
14
C-labeled diuron and chlorotoluron and 2-
14
C-Carbonyl
labeled bromacil together with unlabeled analytical grade chemicals
were used in the study.
The herbicides diuron and bromacil were
supplied by E. I. de Pont de Nemours and Co., whereas chlorotoluron
was provided by Ciba-Geigy Corp.
The chemicals were >99% radiochem-
ically and analytically pure and had specific activities of 3.97, 4.22
and 30.6 pCi mg
-1
for bromacil, diuron and chlorotoluron, respectively.
Purity of the chemicals was determined by thin layer chromatography and
scanning on a Packard 7200 Radiochromatogram.
Herbicide solutions were prepared in distilled acetone.
Two-
milliliters of the acetone solution were added drop-by-drop onto the
surface of 10 g soil samples (water-free equivalent) contained in 50 mL
flasks.
These treatments were designed to give final herbicide concen-
trations of 0.005 and 0.1 pmole g-1 soil which correspond to
approximately 1 and 20 ppm, respectively.
After the acetone was
evaporated, the soil was mixed thoroughly in the flasks and sufficient
amount of water was added to bring the soil moisture to 50% by weight.
The soil was then worked with a spatula to insure thorough mixing.
112
Immediately before incubation began all flasks were weighed,
placed in a controlled temperature water bath, covered with a black
polyethylene sheet to prevent photodecomposition, and connected to an
assembly especially designed to measure CO2 evolution from soil (Fig.
22).
The incubation system was aerated with moist CO2-free air at
60-80 mL min
cylinder.
-1
The purge gas was delivered from a compressed air
.
Evolved CO2 from the flow-through system was absorbed in 50
mL of 0.5 N NaOH.
The experiment was sampled by changing CO2 trapping
solution at intervals ranging from 1 to 7 days during the incubation
period.
Almost every time sampling was performed, weight adjustments
were made and lost moisture was added.
Total CO
2
evolved from the soil was determined by precipitating
the carbonate (Na2CO3) as BaCO3 by adding 5 mL of 10% BaC12.
itates of BaCO
3
Precip-
were filtered through preweighed Whatman glass micro-
fibre filters and washed first with water and then with alcohol.
The
filter discs were dried with a heating lamp and reweighed to determine
the weight of CO2 produced by the soil.
These filter discs were then
placed in glass vials where 2 mL of Handifluor liquid scintillation
counting solution (Mallinckrodt, Inc.) were added and the radioactivity
was measured with a Packard model 3330 Tri-Carb Liquid Scintillation
Spectrometer.
An internal standard was used to correct for the
instrument's counting efficiency.
All treatments were duplicated and
herbicide-free samples were included for each soil as controls.
113
RESULTS AND DISCUSSION
The rates of
14
CO2 production from herbicide-treated soils
incubated at 25, 30, and 35° C are given in Table 7.
The effect of
temperature on the rate of degradation is evident with more rapid
decomposition at the higher temperature.
The rate approximately
tripled with a 10-degree rise in temperature from 25 to 35° C.
The presence of herbicides apparently had no effect on the
activity of soil microorganisms as measured by the total CO2 evolved
(Table 8).
Production of CO2 from the peat was twice that from the
sand for all incubation combinations.
derived
14
CO
2'
Since the weight of herbicide-
would unlikely be detectable with this chemical method,
these quantities of CO2 may be considered to be derived almost
entirely from soil organic materials decomposition.
These differences
in biological activity between the peat and the sand may be considered
a likely factor in the degradation of herbicides.
show that the rate of CO
in temperature.
2
The data (Table 8)
production was doubled by a 10° C increase
This is in agreement with the data reported by
McCormick and Holtbold (1966) and Wolfe and Martin (1974), and
consistent with the generalization that an increase in temperature of
10° C within the tolerance range of the microorganisms results in a
doubling of their activity (Atlas and Bartha, 1981).
The percentage
14
CO2 evolved from the herbicide-treated soil
samples incubated at 25° C is shown in Figs. 23 and 24.
show no sign of a lag phase in the degradation process.
These data
Similar
results were reported by other investigators (Wolf and Martin, 1974;
Zimdahl et al., 1970).
This type of degradation is characteristic of
114
the co-metabolic degradation process where the microorganisms do not
proliferate on the decomposed compound because it does not serve as the
energy and carbon source for the microorganisms.
In such instances
herbicides are metabolized as a result of general microbial activities
which are controlled by the availability of nutrients and moisture
and by the right temperature and pH (Matsumura, 1982).
Microbial co-
metabolism has been suggested for the degradation of many pesticides
including diuron and bromacil (McCormick and Hiltbold, 1966; Lopez and
Kirckwood, 1974; Sheets, 1964, Wolf and Martin, 1974).
The incubation data (Table 7) show that the degradation of
bromacil was very low and essentially unaffected by the herbicide
concentration and soil type.
The rates of diuron and chlorotoluron
degradation were similar, in that the rate of 14CO2 evolution was a
little higher in the sand than in the peat.
In Adkins loamy sand the
rate of loss of the two herbicides was increased as the initial concentration decreased but in the peat the rate was slightly increased as
the initial concentration increased.
This behavior is more likely due
to the effect of sorption on the degradation process.
From the results
presented in Table 8, one expects more degradation in the peat because
it has higher microbial activity than the sand.
However, the sorptive
capacity of the peat for the herbicides has been shown (Chapter I)
to be much higher than the sandy soil.
The present data seem to
indicate that adsorption protected the herbicides from degradation by
reducing the concentration in the soil solution.
Hurle and Walker
(1980) indicated that adsorption however, does not always protect a
chemical from degradation, as was found in this study, nor does it
always lead to increased rates of degradation.
115
Assuming that degradation of all three herbicides followed a
first-order rate law, the incubation data were plotted as the concentration remaining in the soil on a logarithmic scale against time of
incubation in days.
The straight lines in Figs. 25, 26, and 27 are
those of best fit obtained by regression analysis.
The correlation
coefficients of all herbicide treatments were highly significant (r
-0.915), in agreement with the first-order rate law and verifying
that the assumption of first-order was valid (Bromberg, 1980).
The
first-order rate law has been used to interpret degradation of many
pesticides in soil (Gerstl and Yaron, 1983; Hill et al., 1955; Walker,
1976; Walker and Roberts, 1978; Wolf and Martin, 1974; Zimdahl et al.,
The rate constant (K) which was derived from the slopes of
1970).
fitted lines was used to calculate the half-life (t1/2) using the
expression:
t1/2
0.693
The half-life expresses the time it takes the concentration of the
herbicide to fall to one-half its initial value.
The half-lives of
diuron, chlorotoluron, and bromacil calculated using equation (8) are
given in Table 9.
The half-life of diuron ranged from about 200 days
to as high as 4100 days.
Most of these values compare fairly well with
those reported by other investigators (Laskowski et al., 1983; Freed
and Hague, 1973).
The estimated half-lives of chlorotoluron were
slightly shorter than the corresponding values for diuron.
This result
was expected since chlorotoluron is less chlorinated than diuron, hence
it is more susceptible to microbial attack (Matsumura, 1982).
The
half-lives of bromacil in soil determined in this study are consider-
116
ably higher than most of the values observed in other soils (Wolf and
Martin, 1974; Zimdahl et al., 1970; Leistra et al., 1975), but they
compare favorably with some of the values reported by Gerstl and Yaron
(1983).
It should be noted that these half-life values were derived
from the rate of
14
CO2 production from herbicide treated soils under
controlled laboratory conditions.
Freed and Hague (1973) pointed out
that while the half-life is a useful parameter in estimating the loss
of a pesticide from.the soil, it does not tell when the concentration
has been sufficiently reduced so as to have little biological activity.
Since this study concerns itself only with the evolution of
from soil, it provides no information to whether altered but
14
002
14
C-
containing compounds were produced or how much reduction in biological
activity occurred during incubation nor how much of the 14C had been
metabolically bound in the soil.
Martin and Stott (1981) reported
that after one year of incubation the biological activity of bromacil
was largely dissipated, but only 10 to 20% of ring carbon- 2 was
released as CO
2'
They argued that this is an indication that some of
the partially altered or degraded structures are stabilized by incorporation into soil organic materials or by complexing with clays and
metal ions.
Therefore, the half-life value should be recognized as a
useful first approximation but not as a precise figure.
The incubation data were used to obtain the activation energies
of degradation using the Arrhenius equation:
K = A exp (-Ea/RT)
(9)
Where K is the first-order rate constant, T the absolute temperature,
A the frequency factor, R the gas constant, and Ea the activation
energy.
The activation energies shown in Table 10 were derived from
117
the slopes of least squares fit of the logarithm of the rate constant
(K) against the reciprocal of the absolute temperature.
The data
followed the Arrhenius relationship as indicated by the highly significant (r > -0.957) correlation coefficients.
The E
a
values obtained
in this study compare well with those reported in the literature for
several herbicides including bromacil (Walker, 1976; Zimdahl et al.,
1970).
The activation energy is a measure of the amount of energy
molecules must acquire in order to react (to form the activated
complex).
Based on the activation energy values, Zimdahl et al. (1970)
postulated that bromacil degradation in soil would proceed by cleavage
of the carbon-halogen bond, because it is the least-hindered bond and
it has the lowest energy.
They argued that the activation energy
required to initiate degradation of a herbicide should be constant,
dependent mainly on the nature of the bond involved and independent of
the soil environment.
There has been a limited number of studies in
which the activation energies of degradation were determined to verify
such an argument.
The activation energy values obtained in this study
seem to support this generalization however.
118
Table 7.
Percentage
14
14
CO2 evolved per week from
C-chlorotoluron, and
14
14
C-diuron,
C-bromacil treated soil as
affected by temperature and herbicide concentration.
Sand
Peat
Temperature
Ca
C
1
%
°C
14
b
C1
2
CO
2
C
2
Evolved Per Week
Diuron
25
0.12
0.14
0.69
0.35
30
0.22
0.27
1.16
0.77
35
0.41
0.54
2.10
1.18
Chlorotoluron
25
0.21
0.25
0.14
0.38
30
0.40
0.47
0.56
0.91
35
0.78
0.82
1.56
1.40
Bromacil
25
0.09
0.08
0.01
0.03
30
0.08
0.11
0.04
0.05
35
0.06
0.19
0.11
0.08
-1
a Initial herbicide concentration 0.005 pmol g
b Initial herbicide concentration 0.1 pmol g
-1
119
Table 8.
The rate per week of CO2 Production from 10 g Soil at 25,
30, and 35° C.
Soil
Chlorotoluron
Diuron
Temp.
C1
1
C2
2
C
C
1
Sand
C
C
2
Control
1
2
mg CO2 Week-1
°C
Peat
Bromacil
25
13
12
12
12
14
13
14
30
11
12
11
11
14
14
14
35
23
22
23
19
18
16
18
25
6
6
5
6
5
5
6
30
8
9
9
10
9
9
8
35
12
12
11
14
12
11
10
-1
a Initial herbicide concentration 0.005 pmol g
b Initial herbicide concentration 0.1 pmol g
-1
120
Table 9.
Half life (t1/2) of diuron, chlorotoluron, and bromacil in
peat and sandy soils as affected by temperature.
Peat
Temperature
Ca
1
Sand
b
Cl
2
°C
C
2
t, days
Diuron
25
4101
3610
653
1378
30
2166
1805
420
596
35
1131
870
223
400
Chlorotoluron
25
2381
2050
3397
1251
30
1209
1031
804
472
35
617
591
288
370
Bromacil
25
5923
6417
46200
19250
30
7144
4029
10500
9493
35
8885
2596
4076
6417
-1
a Initial herbicide concentration 0.005 pmol g
b Initial herbicide concentration 0.1 umol g-1
c
0.693
where K is the rate constant
121
Table 10.
Activation energy (Ea) for degradation of diuron,
chlorotoluron, and bromacil by soils.
Sand
Peat
Herbicide
Ca
1
C
b
C
2
C1
Ea
Cal mo1-1)
2
a
Diuron
23.23
25.69
18.98
22.65
Chlorotoluron
24.49
22.48
44.63
22.40
Bromacil
-7.44
16.62
44.28
20.06
-1
a Initial herbicide concentration 0.005 umol g
-1
b Initial herbicide concentration 0.1 umol g
E
= Activation Energy = -(slope) (2.303) R
a
Figure 22.
Flow-through soil incubation system used for monitoring degradation of
herbicides.
1.
air-flow meter, 2. drying tube filled with Drierite,
3. soda lime for removing CO, from air, 4. NaOH to absorb CO, from
incoming air, 5. empty flask, 6. water to humidify incoming sir,
7. soil metabolism flask and 8. NaOH for absorbing CO2 evolved.
2.8
I
a)
Q.
4-
0
az:
2.0
)0
s-04
A
O
r-
1
a)
.0
-s-
4-)
=
E
=
6
0.0
,
,
.
0
210
Incubation Period (days)
Figure 23.
14
14
C-diuron-tre#ted soils
CO9 evolved from
Cumulative percentage of
incubated at 25 C. Herbicide concentration: 0.005 mol Kg-'
(11, sand; , peat) and 0.10 mol Kg-' (A, sand;A, peat).
0
28
Figure 24.
14
Cumulative percentage of 14C0 evolved from
C-bromacil-treated
soils incubated at 25° C. Hetibicidelconcentrations: 0.005 pmol
(, sand; II, peat) and 0.10 pmol Kg-' (0, sand; A, peat).
I
0.028
1
1
1
1
1
I
I
1
1
1
1
I
I.
1
0.020
ON
OM
0.010
ml
110
ON
I
0.000
I
1
1
1
1
I
1
1
1
1
1
1
1
i
0
10
20
30
36
Incubation Period (days)
Figure 25.
Effect of temperature on degradation of chlorotoluron
(0.005 pmol g') by the peat soil (4o, 25° C; 0, 30° C; E, 35°C).
1.416
1.415-C")
;
cc
cy)
1.414-"
1.413--
cn
1.412
III
10
20
24
Incubation Period (days)
Figure 26.
Effect of temperature on degradation of bromacil (0.10 pmol g
by the peat soil (, 25° C; o, 30° C;
35° C).
-1
)
1
0.065 _
RIM
0.055-c)
awl
5
0.045--
ce
r-0.035 --
O
0.025--
1
0.015
1
I
I
I
I
I
'
0
10
20
28
Incubation Period (days)
Figure 27.
Effect of temperature on degradation of diuron (0.005 pmol g -1
by loamy sand soil (, 25° C; 0, 30°C; A, 35°C).
)
128
LITERATURE CITED
1.
Ashton, F. M.
p. 117-131.
Persistence and biodegradation of herbicides.
1982.
In F. Matsumura, and C.R. Krishna Murti (ed.) Bio-
degradation of Pesticides.
2.
Atlas, R. M., and R. Bartha.
mentals and applications.
3.
Plenum Press, New York.
Microbial ecology:
1981.
Funda-
Addison-Wesley, Inc., Reading, Mass.
Attaway, H. H., N. D. Camper, and M. J. B. Paynter.
1982.
Anaerobic microbial degradation of diuron by pond sediments.
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Bromberg, J. P.
1980.
Physical Chemistry.
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6.
1959.
The decomposition of
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Adsorption, movement, and
1973.
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Marcell Dekker, New
York.
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Gardiner, J. A., R. C. Rhodes, J. B. Adams, Jr., and E. J.
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1969.
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1983.
Behavior of bromacil and
Adsorption and degradation.
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Am. J. 47:474-478.
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Graham-Bryce, I. J.
p. 621-670.
1981.
The behavior of pesticides in soil.
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Jchn Wiley & Sons.
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Hance, R. J.
1970.
Influence of sorption on the decomposition
p. 92-104.
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Hance, R. J., and R. A. Haynes.
1981.
The kinetics of linuron
and metribuzin decomposition in soil using different laboratory
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12.
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Hill, G. D., J. W. McGahen, H. M. Baker, D. W. Finnerty, and C. W.
Bingeman.
1955.
The fate of substituted urea herbicides in
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13.
Hurle, K., and A. Walker.
p. 83-122.
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1980.
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14.
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17.
Kinetics of linuron
Laskowski, D. A., R. L. Swann, P. J. McCall, and H. D. Biblack.
1983.
16.
1979.
Lopez, C. E., and J. I. Kirckwood.
1974.
Persistence
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Isolation of micro-
organisms from a Texas soil capable of degrading urea herbicides.
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Majka, J. T., and T. L. Lavy.
1977.
Adsorption, mobility, and
degradation of cyanazine and diuron in soils.
Weed Sci. 25:401-
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130
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131
SUMMARY AND CONCLUSIONS
The behavior of six herbicides widely used in agriculture was
studied in two soils:
an organic soil, and a loamy sand soil.
Sorption of diuron, chlorotoluron and bromacil from aqueous solution
was evaluated over a broad range of herbicide concentrations.
effect of temperature on sorption was very small.
The
The results showed
the strong dependence of sorption on the organic carbon (OC) content
of soil.
The organic soil had much higher affinity for the herbicides
than the sand, which contains very low OC content.
Diuron was the most
highly sorbed, followed by chlorotoluron and last was bromacil.
The sorbate parameters:
(K
ow
octanol-water partition coefficient
) and water solubility (S) were found to be good estimators of
sorption constant (Koc) normalized for soil organic carbon content
using emperical equations developed for other classes of pesticides
and pollutants.
It was shown that highly significant relationships
exist between both K
ing Koc values.
ow
and S of these herbicides and their correspond-
It was concluded that such relationships are useful
in predicting sorption of neutral organic herbicides as with other
pesticides and pollutants.
The importance of these relationships is
the realization that sorption of such compounds by soil can be
estimated from knowing sorbate parameters instead of an expensive
experimental determination.
Sorption appeared to protect the herbicides from being degraded.
The degradation rate of diuron and chlorotoluron was higher in the
sand than in the peat but the rate of bromacil degradation was
essentially unaffected by soil type.
The herbicides degraded
132
microbially with no lag phase apparently by a co-metabolism process.
Breakdown of the herbicides was very slow in both soils.
The influence
of temperature on the rate of degradation was much more pronounced than
the affect of soil type or herbicide concentration.
The rate of
14
CO
2
production from treated soils was approximately tripled by a 10-degree
rise in temperature from 25 to 35°C.
Degradation followed the first-
order rate law with estimated half-lives ranged from 223 to 4101 days
for diuron, 288 to 3397 days for chlorotoluron and 2596 to 46200 days
for bromacil.
Infrared spectroscopy (IR) was used to show the involvement of ion
exchange mechanism in the binding of diquat and H-bonding in the
binding of diuron, chlorotoluron and bromacil by soil humic acids (HA)
and water soluble soil organic materials (WSSOM).
Noted changes in
the IR absorption bands suggested the formation of charge transfer
complexes between bromacil, diuron and chlorotoluron and HA and WSSOM
but not between diquat and HA or WSSOM.
The gel filtration chromatography procedure adapted by Hummel and
Dreyer (1962) for the study of binding of small organic ions by
proteins and other macromolecules was used to show the binding of
bromacil, diquat, diuron, chlorotoluron, glyphosate and simazine by
HA and WSSOM.
The study confirmed the suitability of the method for
the study of ionic herbicides and extended its use to the nonionic
herbicides binding by HA.
The chromatographic procedure proved very
successful in showing the existence of binding of the herbicides by
WSSOM.
The association of herbicides with WSSOM has not been studied
before probably because of the lack of a suitable procedure.
In this
study experimental evidence has been provided for the suitability of
133
the Hummel and Dreyer (1962) method and for the existence of association between herbicides and water-dissolved soil organic substances.
These results are important for assessing the mobility and transport
of herbicides and related compounds in soil and water.
134
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APPENDICES
148
Appendix Table 1.
First-order rate constant (K) for degradation of
diuron, chlorotoluron, and bromacil by soils at 25,
30, and 35° C.
Sand
Peat
Temperature
C
Cb
Ca
C1
2
1
2
KC (X 10-4) day-1
°C
DIURON
25
1.69
1.92
10.61
5.03
30
3.20
3.84
16.49
11.63
35
6.13
7.97
31.07
17.32
CHLOROTOLURON
25
2.91
3.38
2.04
5.54
30
5.73
6.72
8.62
14.67
35
11.23
11.73
24.09
18.73
BROMACIL
25
1.17
1.08
0.15
0.36
30
0.97
1.72
0.66
0.73
35
0.78
2.67
1.70
1.08
a
Initial herbicide concentration 0.005 pmol g
b
Initial herbicide concentration 0.1 pmol g
K = Rate Constant = -(slope)(2.303)
-1
-1
4.5
'
11311111
I
-0.5
'
-0.6
2.4
0.4
3.6
Log Ce ( 4 A)
Appendix Figure 1.
Freundlich isotherms for the sorption of bromacil by soil at
sand).
4° C (, peat; 0, sand) and 25° C (0, peat;
3.0
OM
.N1
17)
Ne
2.0
4
OA
IN
ON
INI
Oa
OM
0.0
I
1
-1.4
I
l
I
-0.4
i
1
I
I
2
0.6
I
1
I
I
1.6
I
1
I
2
2.6
Log Ce (pM)
Appendix Figure 2.
Freundlich isotherms for the sorption of chlorotoluron by
soil at 4° C (s, peat; , sand) and 25° C (0, peat; *, sand).
0.24
0.20
0.10
0
100
190
Elution Volume (mL)
Appendix Figure 3.
Elution profile of blue dextran 2000 on Sephadex G-25
gel for measurement of void volume of the column.
7.2
0.7
0.6
0.5
I
0
\ii
.
6.4
0.4
I
0.3
1
_
\
..
0.2
I
If
0.1
0.0
liki
.41. Ole ***
0
100
200
5.4
-
V
11
.
z
+3
SIIIIIMINI
300
400
4.4
500
Elution Volume (mL)
Appendix Figure 4.
Elution profile of chlorotoluron (.) and diquat (., right-Y-axis)
determined on Sephadex G-25 gel.
I'
I
I
t
1
r
I
4.0 -
0.08
3.0
A
0.06
2.0
1.0_
410
I
- - --
0.04
I
:411111.-A-
0.02
OM
0.04
II -1 111V II -- m°
0
1111
100
111111.111119111!
200
,
.
300
2
0.00
400
Elution Volume (mL)
Appendix Figure 5.
Elution profile of glyphosate (A) and simazine (II, right-Y-axis)
determined on Sephadex G-25 gel.
1.20
/IN
ii
1.14
\I al
III Iiiii
lfi ll
\
\ 9. Au
i
.
0.94
0
100
200
300
Elution Volume (m0
Appendix Figure 6.
Elution profile for the measurement of bromacil
binding by humic acid on Sephadex G-25 gel.
0.32
t
I
0.31
. «,
_I,u
0.301/11!,
0.29
.0,
.
.
0.28
1
0
100
i
1
200
i
1011
300
1
400
500
Elution Volume (mL.)
Appendix Figure 7.
Elution profile for the measurement of chlorotoluron
binding by humic acid on Sephadex G-25 gel.
i
1.13
1.01
0
100
200
280
Elution Volume (mL.)
Appendix Figure 8.
Elution profile for the measurement of glyphosate binding
by humic acid on Sephadex G-25 gel.
1.5
13
CL
CL
m
1.2
0
0
0.8
0
04
C)
0.4
';17
*11140
'
0.0
0
a
a---I
10
'
20
36
30
Incubation Period (days)
14
Appendix Figure 9.
14
C-chlorotoluron-treate4
CO, evolved from
Cumulative percentage of
0.005 pmol Kg'
Hetticide,concentrations:
soils incubated at 25° C.
(I, sand; 11, peat) and 0.10 pmol Kg-I (0, sand; 0, peat).
I